Not trying to put every shitposter on ignore makes it easier to report their posts, if you're into that kind of thing.
I'm not incredibly active in reporting, but that's a good reason to ignore the ignore function. Another reason is laziness. Yet another is that curiosity always seems to get the better of me. I suppose it would make it easier to avoid irritation from time to time, but I don't take anything that happens here too seriously. It's just a forum, after all.