As a permanently banned member, I have to disagree. I was an awful poster; only an extremely small portion of my posts were actually worth reading, and even those were of a lesser quality.
I was banned a few days before Christmas of '09, for quoting a spammer who explained how to use a proxy, thus exposing more people to methods of evading the ban system. While this was a stupid thing to do, I thought at the time, and still think today, that it was an irrational response to my offense. But then I realized that I had it coming regardless; I was certainly not a contributing member of the forum. So I took about a 4 month hiatus shortly after being perma-banned, due to lack of interest in remaining on forums I wasn't welcome on (my alternate account had only been recently created, and had little to no meaning to the name). Even after those 4 months, I only came back for a few days and realized I really didn't want to come back. I was too immature before being perma-banned to realize just how immature the majority of the forums could be (don't deny it). So, I stopped coming regularly, and instead just checked back periodically to see if the userbase had "grown up".
I never would have realized any of this had I not been permabanned. The consequences of my stupidity caught up with me, and caught up with me hard.
Regardless, I did feel a yearning to come back about a year after being banned, so I shot a well thought out personal message to Achronos (the one who banned me) explaining the situation in a means similar to I am now. I was by no means demanding, nor was I desperate. I requested that he consider unbanning me, and told him how I would completely understand if he chose to keep me banned. I never got a reply. As promised, I completely understood. (Even if he only missed the PM, I refused to send another; I wasn't going to appear to be begging).
So. tl;dr (if that's still popular here): I was banned a year and a half ago. I deserved it, and I realize that now, but only because I was banned.
All of that being said: For a dedicated spammer, banning won't mean anything; of course they'll have proxies, of course they'll just come right back. But for an average user who commits a wrong-doing, a ban can be a harsh but effective wake-up call that they need to straighten up, or leave.
Anyone curious enough to view my original account can see it here.
Oh, also, is Captain K-Mart still around?
[Edited on 08.17.2011 12:38 PM PDT]