One of the reason I play FF online is for the strict role. When I role a paladin I know that I will be expected to tank. I won't be pigeon holed into another role because square messed up the tanking tree since their is no tree.

But sometimes like you see in EQ2 might be a good idea I'd say.

lets say your an EQ2 paladin well you can spec to improve your soloing, goes purely into defencive when your pushing new content or you can drop some of your defectives move for more DPS or off-tanking. But no mather how you spec you still a tank.

Its really beneficial for healer mostly. While solo questing you can lower your healing in order to get some DPS for soloing. You can do the same while farming content but no mater what your spec is. You'r still a full time healer.