The people that just want to change something in an article to 'poop' aren't going to try to use tor to vandalize and the professional vandals would find it too

The people that just want to change something in an article to 'poop' aren't going to try to use tor to vandalize and the professional vandals would find it too slow (except for making malicious usernames or sleeper accounts) and generally don't have a problem using other proxies or botnet hosts. I'm sure there is some group of people that would like to do something nefarious on wikipedia that are stopped by blocking tor, but I personally think it would more often be someone wanting to sockpuppet rather than vandalize. A few checkusers always pop up to mention that they see tor as a sewage pipe, but making their job easier does not mean the encyclopedia is better off and they would see a strongly biased sample.