Usually I pay them no attention whatsoever.
I joined a little while ago now, I think. Mostly I just read threads without contributing anything myself.
If I intend to post in a thread; I first read the entirety of the first post and every reply, to be absolutely sure I'm up to speed on the happenings of the thread. Far too often, in larger threads, I see people chiming in randomly with rebuttals to arguments settled many pages earlier. Even more often I see people who seemingly read only the topic heading and base their response off of that.
When you read every post in an ongoing thread you often find, as others have said before me, that your point or idea has already been brought up. Sometimes it's already been discussed at length and had its flaws pointed out.
Sometimes you just get to the end and realise it really doesn't matter that much to you anymore. Or the issue has been resolved. Or (admittedly) that the thread has seemingly died and you missed your chance.
This is probably just me though, because I've had too many experiences in the past on other forums where people would flood it all with useless posts just so they can eventually claim they have the highest post count. It makes me worry to post at all sometimes, almost as if doing so is breaking a forum account's virginity.
Though it's just an aversion of mine to "wasting posts", it's not nearly as dramatic as I just made it sound. I almost didn't post in this thread, simply because it didn't seem important enough to do so, though I'm trying to be a little more actively involved these days.
The reason I bring all this up is because by acting the way I do I have a fairly low post count when compared to my join date, and so I take that into account when considering others post counts; or rather by not considering them.
In the end it's the content of the post I focus on; keeping in mind that everybody has bad days, gets sick, or are ignorant about something in this world. If someone's acting badly it doesn't necessarily mean they're always like that.