For the most part the impact on bandwidth should be minimimal. Player posistions are already updated, the formation that those positions create can all be handled client side. There's no reason that the servers have to keep track of anything more than they already do.

I love the idea of synergistic attacks, Cnj Aero + Thm Fire = FireStorm, that's a great idea, I don't know if it should be tied to formations, but it's a great idea none the less. That with melee skillchains where a Pld Spirits Within opens a hole for a Drgs Impulse Drive to make it significantly more accurate. Those things would add some real teamwork aspects. Add to that "Optimal" positioning for different jobs and formations that strenghten different stats would truly add some team strategy and encourage team work, and possibly bring in some reasons to not class stack.