I am sure some will find this addition attractive in the first instance, reality tells me to look for other more effective solutions.
IP addresses being blocked from those who are "unfriendly", are likely to be changed by something smarter than knowing the last IP that tried to do something other than visit your site.
That being said, I see a possible use if it was the exact opposite - allow only from certain IP's.
Intranets, or closed sites.
But thank you for another addition to the J! community