My opinions on this would be that Crusher would technically outrank Troi. I think the original premise of Troi's posistion was that the Enterprise was one of the first starships to carry civillian family members on board so they wanted a counselor on board to help with any psychological issues that would occur. Picard had her on the bridge in more of an advisor's posistion than anything. It never really made a ton of sense why she was on the bridge so often, but I suppose Picard used her almost like an extra set of senors. She'd sense some kind of emotional disturbance and could report it. In later seasons I think they wanted to give her more of a focus on being an officer with a specialty rather than a civillian on the senior staff.