It may seem strange to think of Counselor Troi as a bridge officer, but that is exactly what she was in the first season of the show. Even beyond that she could often be found on the bridge. What business does she have being there? Because as originally established, that was her character's job. What this means, then, is that the later writers of the series like Ron Moore, Brannon Braga, Rick Berman, et al. misunderstood just what Counselor Troi's role was, and in the process created a position of "ship's counselor" which was nothing like what was intended. From definitely the fourth season on, Counselor Troi was the Enterprise's resident therapist. Moore has even joked on commentary tracks about how it was a very '90s idea to bring shrinks along on every starship. That idea might seem to date the show, but in fact I am going to argue that it was NEVER intended to be that way. I know that with a ship full of families it might seem logical to include a psychiatrist, but I think they stretched it too far in making it essentially a ranking Starfleet officer on every ship, and extending it too far to Counselor Troi. But if that's the case, then what exactly was the role of Counselor Troi?
In watching "Encounter at Farpoint" again, you can see that Troi was always a bridge officer. Counselor is still a job description, but what does that job entail? It seems clear to me that her role quite simply was to be an advisor to Captain Picard; that is, to give him counsel (hence, Counselor). She is present at every negotiation in that first episode. I realize that her characters is empathic, and thus would be of great advantage in diplomatic situations to guide the Captain. That's probably why Picard wanted her for this assignment. I do not believe Picard brings her along to see Groppler Zorn just because she knows when he's lying; he does it because it is her job. She's very upfront with Zorn about her Betazoid parentage; she isn't trying to hide anything or give Picard an unfair advantage.
Troi's role in this manner carries through the first season, and even into later seasons a bit. Note that her position as diplomat is integral to an episode like "The Price". So Troi is seated beside Picard to be that extra voice in his ear when a Romulan commander comes onscreen. It's an interesting development for Roddenberry. In his original pilot for Star Trek he had a woman on the bridge, as Executive Officer in fact. The original series' bridge set did not have this three-chair layout. There was one central Captain's chair. Spock, though first officer, was also a science officer and thus stayed at his science station. And all too frequently, McCoy would leave sickbay and come up to the bridge to discuss things with Kirk. As dramatically dynamic as it was to have Kirk, Spock and McCoy all there discussing things on the bridge together, it strains the logic. I mean really, why couldn't McCoy just use the comm system most of those times? Should he really be leaving Nurse Chapel in charge of sickbay all the time? So when Roddenberry created TNG he took the added step of laying the bridge out with those three chairs specifically. Rather than inventing reasons to have a McCoy on the bridge, he invented a role there; the ship's counselor. I know that things changed a bit from the first season to the next. For example, in the first season it seems like Crusher, like McCoy, is the only doctor on-board. With over 1000 people on the ship, that just makes no sense (this point is even addressed in a later episode, "Remember Me"), and by season 2 she had a full staff of doctors. But I believe Troi was always meant to be a bridge officer. Roddenberry recognized that part of what made Kirk a good captain was the input from logical Spock and passionate McCoy and his ability to make decisions from that. Why not, then, put another person on the bridge as advisor to the captain? Somebody whose job it was to remain up-to-date on diplomatic issues with each species; somebody to give the Captain a fresh perspective during negotiations. That is what Troi was there for.
But you may be asking, if that is the case why was Troi reduced to a therapist in later years? This was a slow process, and I believe the first indicator was an episode called "The Neutral Zone". In this episode, three 20th-Century humans are thawed out from cryogenic stasis. When one of them has trouble adjusting, Picard has Troi speak with her and help her locate living family. We learn that Troi does in fact have degrees in psychology and psychiatry. This is the first instance that I can recall of Troi acting as therapist. What unfortunately happened then was the newer writers in the later years began to think that was why Troi was on the ship. It wasn't. McCoy had degrees in psychiatry too. While Troi was useful as therapist it was never meant to be her primary mission.
Like everything else on television, building a world takes time. When we accept that the crew have brought families on-board, the realities of that situation eventually start to be fleshed out. There are schools for the children. There are recreation areas, such as Ten Forward. And yes, it is likely that there will even be a psychologist for the crew to speak with, since mental health is just as important as physical health. I would think though that this would fall under the auspices of the ship's medical staff. Either way, I do not think that it was ever meant to be Troi, or really that it should ever have been a Starfleet position. I know that Troi wears a blue uniform, signaling someone in the medical or science area. This however could simply be due to her degrees, and frankly she spent so much of the first six seasons in catsuits that the uniform was secondary to what her role was. After the pilot, we don't see her in uniform again I think until "Chain of Command" six years later.
The biggest blow to the original idea of ship's counselor came in the fourth season episode, "The Loss". Here Troi has been firmly established as the resident shrink on the Enterprise with office hours and regular appointments. We watch her in sessions with crewmen. This episode I think is the single most destructive to what her character was supposed to be. From here it became clear to everyone that Troi's role was to be the touchy-feely one helping Worf and his son connect or helping Picard through personal crises. As episodes progressed from there, she seemed to have a certain ineptitude for the goings-on of the bridge. The movies haven't helped, since she's been at the helm several times and both involved crashing the ship.
Perhaps the most unfortunate result of Troi's essential demotion was the collateral damage it inflicted throughout the Star Trek universe. The role of "ship's counselor" became one of resident therapist, and one which every ship was assigned. I've heard this point as one of derision. And it's certain high-profile writers for the show who I think are most responsible for this misconception. The counselor issue continued on Deep Space Nine, where a number of the better TNG writers worked. When Ezri was added, they wrote the character as a young ship's counselor. And what did that mean? It meant she went around trying to be a good therapist to people. A series that did fine without this sort of thing for six years, suddenly got back into "Counselor Troi" mode by having Ezri try to guide her superior officers by helping them recognize their repressed feelings or work through their claustrophobia. And while Voyager did not have a visible ship's counselor in that manner and was able to avoid this pitfall, every now and then Troi would appear on the show and once again she would be trotted out solely as Reginald Barclay's personal therapist. This perversion of who and what her character is to me is the fault of writers like Ron Moore and Brannon Braga. Note that in the Michael Piller-penned Star Trek: Insurrection, Troi is back to advising Picard on diplomatic issues. Actually, I should point out in general that Troi is back on the bridge on the Enterprise-E in the movies.
All television is a product of its time, and there will always be things that date it. But it has started to really bother me when people use the Ship's Counselor as a reason to slander Trek. Quite frankly, the prevailing concept of what a Ship's Counselor was is wrong. Counselor Troi was a capable Starfleet officer assigned to counsel the captain, not to take care of the entire ship. The clearest evidence of this rests on that main set that was seen in every episode; the unnecessary shift in Troi's character is reflected in that constantly empty seat by Picard.