They've already confirmed (and shown) the ability to play as your entire party, so no worries there, I think. I doubt they'd build a system around that ATB gauge that didn't have some advantage to switching between people.
being able to switch party members isnt enough i want to control each party members actions. i'm thinking the HUD isnt finished yet though. it says limit on the right but clearly the guage on the left in the limit bar.