Well, see...I raise an eyebrow with your L4D comparison. I admit I lack a 360 so I've never played L4D but I assume like MW2 single player is not why people buy/bought it, whereas the same can not be said about Splinter Cell games.
I admit, I'm biased against adding multiplayer in games that's obviously a single player affair but the way I see it, this is a game about Sam Fischer, not Sam Fischer and chums. I was against it when Naughty Dog added mp to Uncharted 2 but I admit I actually enjoy it.
But if they then announce in Uncharted 3 they won't have multiplayer for whatever reason, I don't think that's a good enough reason to boycott the game.
I guess it's just me refusing to believe your sole reason for looking forward to Splinter Cell predominantly a single player affair was the multi-player, because boycotting it suggests this to be the case.