Having a therapist onboard the vessel makes sense (honestly, given the number of people aboard the Enterpirse-D, having several would be ideal).
Having Troi on the bridge never made much sense to me, though. One would think you'd want her to be busy being a therapist, which we're supposed to believe she has time to do in addition to her duties as part of the senior staff.
I've always believed that was more of a Picard thing due to Troi having empathic abilities that would give him
an advantage when dealing with people. On other
Galaxy-class ships, counselors are stationed below decks, with that third command chair on the bridge occupied by the second officer or even not there at all, IMO.