Permanent bans. Except for egregious cases, ALL bans should be for a certain number of hours to cool the situation down, and then let it go.
You would be surprised at what portion of bans you would consider "egregious cases". Spambots and regular spammers, for one, but also racists, sexists, transphobes, homophobes - basically all manners of bigot. Bigots made up about 97% of my permabans when I was a mod. The other 3% were spammers, serial trolls, and people who were obviously on an alt and ban evading.
People can read into a comment a whole lot of stuff that isn't there. And it is rather easy for a mod to hit the ban button and think the problem will go away. It doesn't. It only gets worse. And the fact that Reddit mods were quick to hit the ban and then silence inquiries as to why someone was banned only aggravates the situation further. To the mod, they think its over. It isn't, because the person will come back under a new account and will be even more aggressive next time.
That can all be eliminated with a simple 24 hour ban from that particular sub. By then the thread is probably dead and people have moved on.