It might be OK for them to put Sonic in a Smash Bros. game, mainly because you could smack him around real good with Mario. But actually putting them in a game together... no way. It just wouldn't work. Sonic is all about image, and trying to look cool, but Mario is about chivalry, peace and love. Not to mention constantly thwarting evil... I think Sonic's no match for Mario anyway.
Also, they couldn't put sonic in a Smash Bros. game anyway, because Nintendo doesn't own the rights to him, or have that sort of agreement.
"Tonight, I`ll make an old family recipe. Pickled dandelions with barnacles in a diesel marinade!" - Prof. E. Gadd