>>63
I've historically used IPs to ban entire ISPs (Amazon, to be precise) during massive attacks on the site.
At some point this guy was spamming Anita Sarkeesian videos on then-/gg/ and every IP was coming from an Amazon server. After a few hours one-at-a-time bans, I shut down literally hundred of thousands of IPs that belonged to the Amazon cluster for a few days. This stopped the attack.
Hashing IPs would debilitate global moderation.
There's also an issue that IPs are VERY, VERY easy to make rainbow tables for. There is a finite number of IPs and in IPv4 that range is actually really small comparatively. If a malicious entity or government seized the software to get an IP, they could build a rainbow table in a matter of hours that effectively broke the protection.
The only other thing I can think of is enhancing my autopruner to be able to remove IP data after X days as well.