My opinion on the topic isn't definitive by any means, but I think a lot of projects would do just fine without allowing arbitrary hyperlinks to be added by non-admins.
I think being able to link to related issues and link into the code is still important, for example.
It's certainly a trade off, but spammers want it to be rendered as a link.
Just so that I can follow - URLs posted by non-admins should not render as HTML URLs at all? Wouldn't that be quite limiting for OSS project members for example?