Im sure it would make others buy it, due to the fact they might have silver membership and would still like to play with friends. I will buy it either way.
I <3 splitscreen. Because, say, there are two people in a household with a Live account and a game is bought and because you can't play splitscreen co-op one person ends up watching the other person play through the game... now seriously, I have a hard time playing a game after watching someone play through it, because it feels like cheating, yes, I mean it, cheating. There aren't any surprises and you get to see (while there may be some improvements on the second gamer's part...) the right way to do everything. So a game with splitscreen makes me happy, because I can play through it with the other person and we go through everything all at once. Now, I know, not every game is meant to be a co-op game, but if you're going to add in some sort of say... co-op games you can play (MW2) later on, then why not have it from the beginning? Or any multiplayer online game... I'm not going to argue that some games would be absolutely retarded this way, but there are quite a few games lately that could have had it from the beginning of the campaign, etc.
Yea my bro bought MW 2 recently and we were depressed that you can only play split screen in spec ops and local multi player, We both have an xbox live account and MW2 isnt the type of game that you would go and buy another copy just to play xbox live together.