Just because it's "easy to evade" doesn't mean it won't stop people who are less tech savvy from evading.

And besides, only the most persistent ban evaders would try to do so, and the only one we know of is TED.

Even so, we should still be prepared for other rule breakers that aren't TED, as IP banning would stop the person from signing up multiple times if they don't have any equipment or access to another computer ("proxies" count as equipment), which the former would only work if they aren't savvy enough with internet technology.

In the end, IP banning would tell them we REALLY don't want their kind, and thus, if they want to be persistent, they'd try to get around the IP banning.