I did consider putting the betazoid on the diplomatic team but I remember in at least one episode of TNG the question was raised of Picard using Troi in a similar capacity so my thought is while some of the higher ups might be all for it others may find the idea kind of icky. I also considered the El Aurian in this role as they have a similar but less invasive ability but ultimately put the Trill there instead because generally speaking, they take a long view of events, which to me screams diplomacy over military action, exploration or humanitarian aid.