I fully agree that should be not needed registration or login to browse the side or download plugin
But i believe will be better request login to reply to messages, that will reduce Spam and also confusion
Is confusing read several messages of "Anonymous" posted in the same thread,where several "Anonymous" reply to each other or maybe the same reply to himself.
And obviously for a spammer do not make much differences open new topic or reply, only difference is a disadvantage for this side:
is more easy notice and and delete a new topic that is spam, that spam hidden in reply as "nice work thanks blog (Referral link hidden in "blog")
Is also a standard in many similar side, you may read, you may download,BUT if you want post, does not matter if a new thread or a reply you have to login...
registration is free and quick
Another point the "Report this message" button went lost, so is now impossible report spam and so many do not resist the tentation to reply to spammer (as here http://registry.gimp.org/node/14287#comment-2370)
..i also replied once to a spammer but then i was not 100% certain that was spam and not just a very dumb reply