We came up with this solution in response to all the problems we were seeing from problem users on our social networking app called Nearby. For us, it was worth alienating the small number of legitimate TOR users. The results for us have been overwhelmingly positive. I definitely agree that it isn't for everyone though and is something that should be carefully considered before implemented.
I'd be interested to know what HN and Reddit do.
I can't access Tor's site from where I am right now, but they also have a post about blocking traffic from exit nodes.
IRC servers also have to make the decision whether to block or not - I'd also be interested in opinions on this from IRCops.