I'll be honest, If it were one of my boards, I wouldn't do it, but then my boards each have teams of Administrators and Moderators (I think there are Mods here but there identities are secret) and while they do get spammed on occasion from bots and disgruntled members there is always someone with the power - to delete and ban IP's - online.
However, populations are different. My largest board site having 30 Admins and 15 Mods overseeing >100,000 members.
Of course, the main reason I wouldn't do it is because it would stop Spiders visiting my site, which in turn would affect my Google ranking and ultimatly hurt my pocket as I recieve revenue from affiliated shops like Amazon and click through revenue from Google Ads. No search engine results mean less visitors to me and less visitors mean no commision from either of these sources.
AH doesn't require generated revenue from these sources so it is a different kettle of fish.