Not to be mean or anything, but if you've paid attention recently you'd have noticed that the spam is all of the kind for people advertising a product or saying something like "Hey, so you guys gonna vote for *insert cause here*?" Those are basically the only posts that have happened that I've seen, so I don't see why people just can't sign up for an account... although I suppose that would mean they'd just sign up and then spam that stuff. Maybe.
I don't know, now, actually, because I'd never thought about that whole "post three times and join" thingy. I've always been invited to forums, so I've never had to post just to see what the people were like.
In a general sense though, I wouldn't really do that- I tend to lurk in the forums a bit just to see what's going on and then I'll sign up for an account and post.
Eh. It could go either way.