What makes you think that karma is meaningless? I find it meaningful.

Sometimes I read a troll-like post, and check out the user that wrote it. If they have 3000 karma, it is likely that I am missing their point, and need to think some more. If they have -10 karma, then they are probably just a troll.

I know it's "bad" to judge someone by a number... but we do it because it is usually helpful.

What you describe is the intended, valid use of the metric. What I am complaining about is people who care on a personal level about what their own karma is, and who do things just to inflate that number. Facebook/Myspace "friends" whom you don't actually know? "Vote up if *" posts? Lolcat or comic or whatever posts? I know HN is (mostly) blissfully free of these things, but they are a direct consequence of the karma system.