When I used to make money on a forum (I don't anymore) through Google adsense, newsletters drove big traffic to the site and therefore juiced the ad revenue. Therefore having a large e-mail list of users was beneficial monetarily. OTOH, folks who sign up just for the purpose of gaining access to something as trivial as the ability to view attachments are probably more likely to flag your newsletter as spam, which of course is not good. There are tradeoffs you have to weigh on either side of this question. I'm no longer after eyeballs, a stance that makes me happier.