My static IP was erroneously blocked from Wikipedia as some previous owner used it to run open proxies or the like. Never had any problems anywhere else. It happens.
It's not a defence, it's just reality. Abuse is real. Using IP addresses to combat abuse is very broken – I too had carrier-grade NAT when I lived in a developing country in the past – but it does work. Abuse prevention is hard, but very much required. Abuse prevention with zero false positives is nearly impossible with the current state of technology. So yes, "it happens" because all of this is hard. You can pretend the problem doesn't exist, but that's denying reality.