I think this is wasting weapons that could be used for future jobs. :B
Also, with the system you're suggesting, how would you translate some of these ideas? Like Bard turning into a healer when it has no healing skill? Or turning Dragoon into a tank when it has no defensive cooldowns other then Keen Flurry (which boosts Parry) and Marauder's Foresight? And then you have Tanks-turned-DPS that have all these defensive skills and cooldowns. Not to mention changing all the animations.