But not a lot of people enjoy the meta game enough to train up new Pokemon from scratch, that would be tedious and not very rewarding.
Well, that's the way I like to play and I would personally be dissapointed if the battle frontier became so easy you could beat it with whatever your main team was without any changes whatsoever. I understand others play differently, but those are just my desires.
Besides, with the new experience share in X/Y, and if we are able to earn EXP Points from secret bases, I think training up new Pokemon and teams will be a lot easier in ORAS, not to mention the new breeding mechanics introduced. I don't think the battle frontier should require IV breeding to beat, but with horde battles, EV training is easier than ever before. Not to mention, is super training comes back in ORAS, we'll be able to see EV's again.
A lot of people complained about how easy X/Y was to play in the main campaign, and it didn't get any harder in the post game. The battle frontier is the perfect place to give us a challenge. It think its almost designed to make you use multiple Pokemon. Personally, I don't think its possible to keep it challenging whilst still making it easy enough to use only the six main pokemon you used as an in game team. I think using your in game team will be fine at first, and could probably get you to silver mode and get you some base BP to spend on useful items and TM's, but Gold I suspect will require more strategy.