How couldn't it not be bad? By sticking Snake in there they've made the game much more violent and have raised the ESRB Rating significantly. Characters in Nintendo games aren't suppose to be using nothing but machine guns, realistic explosives like grenades, and other realistic weapons. Besides, Snake would've been better off being in Halo or a game where he'd actually fit in.

As for Sonic he's overrated and will take away from the Nintendo feel of the game. Too many people will be using him which will destroy the purpose of the Smash Bros. series: having Nintendo characters battle it out and see who'll be victorious. If everyone starts using Sonic and the third party characters that'll no longer be the case and it'll just be some game where everyone will use anyone that isn't from Nintendo which will ruin the Smash Bros. series. You can go ahead and say that people won't be bad enough to use only 3rd party characters but it won't be true. Having no 3rd party characters will make sure that this has absolutely no chance of happening and will ensure that the Super Smash Bros. series won't be heading downhill as a result from them.

You could stick them all in some sort of game that's not Nintendo but uses the same system as the Smash Bros. series and I'd be more than okay with it. But having them with Nintendo characters is wrong and will destroy the Smash Bros. feel to the game.