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.
"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