I agree with theymos. We want to make it as easy as possible for people to find out and become involved with bitcoin, not make people jump through hoops to become part of the community.
I know there was some spamming on threads early yesterday, but has this been common up until now? Is it really enough of a problem at the moment that we need to take such a measure? Either way, using captchas would be a better way to stop spam than requiring a login.
As for "protecting members opinions", this is intended to be a public forum. If you don't want your posts indexed by a search engine, then I can only respectfully suggest that you don't post anything...