I got into promoting cam sites because with file-sharing, huge hard drives, portable DVD players, ripping software everywhere etc etc. selling "static" content just seemed less profitable long-term. (ie Free content was killing paid content)
Live cams offer real-time, 2-way communication/interaction that can't be burned onto a CD or shared on Kazaa.
So I'm far from an authority on this topic, but in my area of work a teaser chat is designed to lure the viewer into paying. There are still far too many "sitters" in each chat room but the ones that open the wallet often open it wide.
So .... in a sense, it's offering free softcore (clothes on/group chat) to lure into hardcore (live sex or solo/1-on-1)
(Gee, Izz, I hope I didn't stray toooo far from the topic???)
Then again, the toolbars that offer access to DVD rips seem to be along the lines of offering free porn - for a price (the software). Kind of like selling pick-axes to hopeful gold-miners ... so there are concepts that can be profitable by offering "free" content
(ok, that's a little more on topic...)