It depends on the game. Halo and CoD are MP franchises with solid single-player campaigns. Some of the best combinations of the two around. BioShock simply wouldn't work, make sense, or be as great with a multi-player component. It just doesn't fit the environment, story, or anything. While the Splinter Cell series has multi-player, I view it as a BioShock. It doesn't make sense to add multi-player components to stealth/man on the run games while the competitive nature of a military/war FPS naturally lends itself to a great (and long-lasting) multiplayer experience.
The same applies to platformers. I am a strong opponent of multi-player in platformers. If we start seeing multi components in games like SMG, Banjo, R&C, etc., I fear the game length and single-player components will go the way of the TPS/FPS genre. If I start seeing 10 hour or less platformers, I think my head will explode. I put up with it in FPS because most modern FPS games that are worth a damn have both a co-op and a multi-player component that doesn't get old... I don't see that happening in the platformer genre. There will never be a super-competitive platformer that has the multiplayer legs of a CoD or Halo.
In short, I love 'em both. But there is a time and a place for each experience. Some genres lend themselves well to MP, others lend themselves better to SP. Very few have a decent mix of both.
Or check out my new webcomic: http://selfcentent.com/
