It would just seem really frustrating for users who want to participate in account based sites (and really, what sites aren't) to find themselves kneejerk blocked at the network level.
Tor is slow, there's really no claim to be made that Tor is capable of literally DoSing a site. Only one for cranks and spam and abuse. For which higher layer filters/moderation can work, as does simply closing the offending account.
So when a way of running an exit appears, such as perhaps this one, that might mitigate against the kneejerk for those 'good' users that care to find them... myself, I'd tend to give the world the benefit of the doubt and run one [1]. It's not like anyone who already disbelieves in Tor/anon/etc enough to block it is going to donate to Tor for providing a nice [DNS/consensus] blocklist or stand up amongst their peers and say 'Tor is some really nice guys'.
As a site admin, Tor is but a speck of trouble compared to the rest of the IP space. Put yourself in the shoes of a public service... Facebook, Gmail, forum, whatever... though Tor might bring some rarer serious issues, that's nothing compared to the ticket counts of the general moronic public.
[1] Caveat testing to be done regarding proof for those needing it via Exonerator, etc.