I can definitely understand if a game with a large multiplayer focus has a lot of its achievements reserved for that area, and I think MW2 would actually have been well justified in having even a great portion of its achievements allocated to the multiplayer side of things.
On the other hand, a lot of games that have multiplayer just for its own sake often have a great number of useless achievements locked down for it. Condemned 2, anybody? What the hell?
Another issue they tend to present is that achievements for multiplayer aspects of a game are often much, much harder, time-consuming or, worst, just plain luck-based than their single-player counterparts.
I don't think it matters all that much in the end, though. On one hand, if you enjoy multiplayer, then you probably won't mind, in any case. But on the other hand, if you don't play multiplayer.... then why do you care if random people on the Internet see that you got all the achievements for a given game? And if it isn't for them, then who? Yourself? Is the actual fun you get from games less important to you than the gamerscore you get from them?
I don't go for multiplayer achievements, because nabbing all the achievements for a game isn't important in and of itself and I'm not going to slog through something I don't want to for their sake. On the other hand, I have cleared out the achievements on a few games because the achievements gave me extra things to work towards in an experience I already found fun and enjoyable. In NO case have I ever cared about gamerscore.
And I think a lot of people feel the same way about it as I do, and play accordingly.