To me, it is like having a shop and banning everyone whose phone has internet capability as they may go online and read something bad about me or my products.
I love the google toolbar. If I go to a site and it says I am banned because I am using it, I will not uninstall it, I will just assume that the website has something to hide and not give them my business.
Side wiki will get spammed to death. It will take about 30 minutes to make a tool that spams it automatically (just use Imacro with the CSV feature). I seriously doubt that many people will pay attention to what it says, or even bother to look at it.
Google must have thought about this before they released it, so I am sure they have some sort of game plan. I would love to see them remove this feature but I can't see them backing down based on website owners opinions. Googles business model revolves around the end user, and if the end user says he likes it, then it will most likely stay.
All I am saying is take a second to think before you take any actions that will lose you money or you will end up shooting yourself in the foot for no reason.