Just because one game in their entire line-up goes a Hack & Slash routine, doesn't mean that they'll instantly use it for every other future game coming out. XIII seems to be a sort of mini-series of FF all together in a little universe of its own, it's very likely they'll use this Hack & Slash for Lightning Returns, but return or do a different battle mechanic for other FF titles.
VI - IX had a party you could customize, but only outside of battle. X gave us the option to exhange our active party members during the battle - somewhat necessary, as that was the only way to get them AP, actually.... - and then returned to being able to only change party members outside of battles in XIII (After having a set party for majority of the storyline... I dunno about XII)
So, it is likely they are simply trying something new for LR. Actually, most games would be boring if developers didn't test and try out different things.
C-A --- "If sense were common, everyone would have it." The gamer who isn't afraid of admitting to disliking the so-called Best Games of their Generation