I'm a carrier DPS so I'm looking at this from a person who Mains Dragoon and is soon to add Ninja and Bard to the list of "Main Jobs" so I'm looking at this from a "How would we (The career DPS that either don't or prefur not to tank) benefit from these ideas?" perspective. And giving us a small responcibility like Ogulbuk's "Emegency Shift" idea would give us the credit we diserve and make tanking and healing less stressful if something went wrong. THUS tempting more people to give healing or tanking a shot because they know some one has their back.
This would allow for more comradery between players as well because it would bring us to a Unit level of operations instead of 4-24 individuals just beating their heads against something. It could also allow for more dynamic game play as a system like this would be skill dependent rather than just "I'm going Tank!!! Leeerooooooy Jeeeenkiiiins!!!!" and still dieing because you have WAAAAY less armor than that PLD does.
So all in all I'm interested in it because it's the best idea I've heared yet, and one I'm sure would interest Director Yoshida as well, but simply needs to be examined for abuse potential so apropriate limits can be set. For example, ONLY being able to shift to Dragon Knight when the Tank(s) go down and not being able to shit back for saaaay around 5 minutes maybe so that Raise/Ressurection can be cast and the Weakness Debuff to time out. Ex. I shift to Dragon Knight when the Warrior hits the dirt and cover him as off tank untill he's rezed. Then when he gets up he throws Beserker to cover my DPS position untill weakness wears off and we switch back.
Think SAO when they yell "Now Switch out!!!". It would also make for an obsean amount of synergy in statics when every one is in perfect synch with how the others play and can get the "Switch" timing absolutly perfect.