Third Nacelle wrote:
Guinan's main purpose as a character is to be someone outside the chain-of-command to whom the main characters can confide or get her advice. She also provides Picard with a friend who's not under his command. By virtue of her position and her age, she's able to see things from the outside and understand the big picture in ways the other characters can't.
Thing is, when you put it like that, it just shows up what an utter failure Troi was as a character. Most of that - between her medical-dept job and her empathic powers - should have been her role, and if she'd been better-handled they wouldn't have had a gap for Guinan!
Sometimes man will tell his bartender things he'll never tell his doctor.
Troi could never fill the role of Guinan because Troi is Starfleet through and through, the problems the crew faces are her problems. She is not the outsider that Guinan is. Geordi would never go to Troi for advice on picking up women, Data would not depend on Troi to teach Lal about humanity.
Picard goes to Deanna when he needs close-up focus on an issue, he goes to Guinan when he needs to back away from the trees to see the forest, when he needs perspective.