You're right, but I don't have a clear picture what the typical hellbanned user looks like. There are spam bots submitting stories, but you don't see them in discussions. Hellbanned users you actually meet in threads are, at least as best as I can recollect, most often posting normal stuff that wouldn't rank as low quality. Sometimes I click on their history to see where things went wrong and it's usually not hard to find this one post that ended their account. It doesn't matter if all the rest of it was pretty good, that's all it takes: one glitch.

I'm not sure if I'd say this is a good system or not. Maybe it would be more productive to have a time limit on bans that result from one-time offenses.