Troi definatly has a purpose, she went through Starfleet Academy, although she was attached to the Sciences Department, she would have had all the Tactical and Starship Operations Training like everyone else serving aboard, as a LT Commander, she would have earned that rank and even though people often joke about her crashing the Enterprise :P she obviously had piloting skills to take over at the helm in the first place
We also saw Troi take the Bridge Officers test, qualifying her as a full Commander (and perhaps allowing her the option to pursue Starship Command in the future)
Obviously people in the 24th century still require a Councellor, especially on Starships that carry families, Troi's purpose was to liase with the senior staff (since the senior staff were also department heads) about the officers and crewmen under their command
So in hindsight, Troi did have an important purpose
I agree totally. And this brings me back to the point I made earlier about Troi not knowing what an antimatter breach was. Even though she had graduated the academy. Sometimes her purpose was to act stupid so the crew could explain it to her and the audience. Ordinarily they wouldnt need to explain it to crew members because they would know.
Troi also had a really bad habit of stating the obvious to a point where Sigourneys Weavers character in Galaxy Quest seems to have more purpose on that bridge. I can't think of an example right now but next time I watch TNG and spot one i'll post it.