I've always been a fan of skill trees and people using the same job in different ways. Unfortunately, though, in MMOs... people are going to find the one "best" route and pressure everyone into using that. So even with options, things just end up being linear anyways.

I can see it now. *enters dungeon in duty finder* Rage quitting perfectionist: "What?! You didn't go THIS branch of skill tree?" *quits immediately without even starting the fight.*