See: "Also, when we block an IP range for abuse, innocent third parties can be affected."

Although they refer to IP ranges, the same principle applies on a smaller scale to a single IP address: (1) dynamic IP addresses get reallocated, and (2) entire buildings (universities, libraries, hotels, etc.) might share a single IP address.

Aside from accidentally affecting innocent users, you also open up the possibility of a DOS attack: the attacker just has to abuse the service from an IP address that he wants to deny access to.