Since tags are easier to make than they are to block, it's safer to ignore a user than a tag. And jerkers are anything but consistent with tags. #circlejerk, #redditjerk, #test... it goes on and on and on. Why would I bother with someone whose interests run so completely counter to what I want to use this site for? Why wouldn't I throw the baby out with the bathwater when there are so many babies and so much bathwater?
Well, I'm talking to you right now and I have you ignored. It's not like I'll never see these people- we'll have some posts in common and when someone seems like not a jerk and I have them ignored, I'll look into their shares and if they're overwhelmingly sharing good content, then I'll follow them and think I was wrong.