I feel the same way, I mean obviously their intention is to force people to raid in mixed groups, but it also forces people to play jobs they might not want to.
Right there, that's the problem that bothers me, it's forced. Not an incentive, if they wanted an incentive for people to play other jobs, why not make it charge faster? It basically punishes groups for allowing people to play the jobs they enjoy. Now for certain fights (Twintania as you already mentioned and I forsee Extremes being the same) the groups people use will have to either be altered, or people will have to relearn/adjust parts of the fight.
I guess it will depend on how severe the charge reduction is, but I can see it being a lot harder.