allow me to explain why i play the games i have for PS3 and xbox 360 (this is not my complete collection FYI)
Battlefield bad company 2: only for multiplayer. the campaign is stupid and pointless, although it is useful for warming up so that i can blow a bunch of holes in everyone online (i gotta be at my best to do my best, eh?) My friend has bad company 2 as well, but he has never been online in it and is always going on about how amazing and epic the campaign is, and i gotta tell ya, it really annoys me. but hey, all he has is a PS2 and a PS3, I have a PS3, an Xbox 360, a Mac with Tf2 on it and WII (though i regret buying it), what does he know about gaming?
Black ops: i don't like it anymore. the graphics look muggy and stupid, the storyline is even more stupid (I was expecting it to tell the story of the life of a soldier fighting in the vietnam war and have lots of stealth and stuff, not the story of a schizophrenic former soldier guy who got betrayed and brainwashed by his best friend in prison and then proceeds to slaughter a bunch of communist russians, which is bloody typical these days, killing communist russians i mean. just set it in present day and it would be a generic shooter) zombies mode is cool, i guess, but the fact that the zombies all sound like possessed demonic asylum patients and not zombies really tends to get on my nerves.
Mass effect 2: it only has campaign, and that's alright with me. while some parts bug me (getting sufficient good karma so that i can ensure the survival of my team and the loyalty of all of them, mining for stuff i can use to upgrade the ship, etc) i absolutely love the mass effect series for its epic storyline. it would be cool if it had multiplayer, though. perhaps you could play as any character from the series (humanoid characters only, no collector general, hanar, volus, rachni or reapers) use most of the guns, and have big 8 vs. 8 team deathmatches and stuff!
rAge: haven't played it yet, but i bought it for the single player.
CoD 4: loved the campaign, multiplayer is good too.
Gears of war 3: I once played nothing but gears of war 2 for an entire month, maybe more. that's how much i love gears of war, although i hated the first one for its dumb story (the story was literally the story of four freakishly muscular power armour-clad soldiers trying to blow up the locust's home turf, but it was mostly focused around killing berserkers, the stupidly short and unsatisfying chainsaw bayonet eviscerations, and closing e-holes with frag grenades) the good thing is that gears 2 and gears 3 actually have a plot and aren't just boring linear slaughterfests, gears 3 has one moment where you encounter a berserker, gears 2 has none, and nobody even cares about closing e-holes by gears 3! oh yes, and the multiplayer is fan-tas-tic!
so their you have it. some of my shooters i bought for single player (most of them, really) and some of them for multiplayer!