The thing that converted me to thinking about .onion sites was the fact that the BBC has one. It might be a good idea, only if the forum was in read-only mode only. No logins. Then no such thing as bad marketing like we always hear from the experts!I'd be all for it, but don't see a real 'need'. If it was easy and didn't use many resources, I guess there's no harm.
MIGHT be an issue with the site functioning properly because of disabled javascript etc. and issues with payments, but not really familiar with .onion web design. Maybe it's just the choice of using a Tor browser that has the limitations.