Oh-- and the other thing that happens is that occasionally, spammers are looking to exceed some number of posts so that they 'unlock' forum features-- like member lists, which (hopefully, from their perspective) include e-mail addresses. Selling them is worth some money.
Of course, in this arms race, most forum administrators are now wise to this, and even with hundreds of posts, that kind of info just isn't available to regular members of a forum.
At my site, we still
allow members to share their e-mail addresses, but the threshold for seeing them is pretty significant-- and the threshold for viewing some of the private places in the forum is, as well. Very few of them make it through to that level.
But a few try every month.