I suppose they could impose a sort of reverse Aion thing where your stats temporarily decrease when you move. I'm not a fan of your solution. I usually use my Arcanist when soloing anyways. Nothing outside dungeons really threatens me anyways, so I might as well keep the few cross class slots open for things like the Truth of X skills.

My current issue with your plan is that what skill you say (knowing the fight and planning your skills to suit it) will only really apply to the first few people to come across the fight as any given class. Most of the rest will just look up a guide about how they should set up before the fight, and then just spend the fight hitting buttons. I'm sure you have the best of intentions, but I can see your solution just ending with industrialized laziness. My FC grinds me up so I get the skills, tells me which to set where for what fight, and then I'm basically walking around as a fancy beat stick.