I don't think it has to do with seriousness, but fairness. I mean...I tried playing last night after Spec Oping and I quit after two matches. The first one I joined, everyone was camping and I ran two feet after spawning and got killed, then I was killed 4 more times with no kills at the end of the match. :/
And the second match, I joined in and was killed instantly by the 1887's, then proceeded to die three more times before quitting early. I know they're not as powerful, but it still isn't great to be sitting around a corner and some dude just blast you with them in the face. XD
BUT...I did have one good match earlier in the day at my family's house. I got like...a 15 killstreak. XD; That obviously didn't last long. :)
Either way, I think I'm done playing MW2 in random matches and I'll probably just stick to playing with friends when they invite me to play. Cause I don't seem to care as much about dying when I'm laughing with buddies.