I have never received one of those ads yet but I've read some about them. I had no idea they aren't clickable though. That would seem to make them almost pointless.
The only way I could see it being very effective, would be to pop a message like:
"WARNING: Your system has been compromised. Type
www.blahblah.com into your browser immediately to fix this."
Have an all in one firewall/evidence eraser type thing for sale at that site.
But for porn sites. I can't imagine too many people typing in a porn URL from one of those. Not like it's real tough to find free porn these days. :P