Well, in Nintendo's defense, even though I didn't care too much for the game itself because of wicked crab handage, Metroid Prime Hunters had a pretty robust online interface going on. But yeah, the other WiFi enabled Nintendo games I've played haven't really had the best online.
But lag or no, they'd definitely lose some sales if they were to exclude online play. Everyone I know aside from myself and a handful of real life friends are buying the game for the sole purpose of online play. And even if there is lag, I don't play the game for tournament stuff anyways; for me Smash Bros. is first and foremost an ode to Nintendo and a wetdream for Nintendo fans, then a party game, and then a serious tournament game. I know most of the people on these boards are hardcore gamers and whatnot, but Smash Bros. is a game that appeals to all kinds of gamers, including the ones that don't mind a pinch of lag in their online soup. So yeah, definitely beneficial to have some kind of online stuff in this, especially if Nintendo plans on not being bombed by some crazy fan if they decided to not include it.