Well, the softcore/glamour and non-nude are two seperate beasts. I can easily see how a site like
http://www.macandbumble.com/ can convert and retain members. It's high end "classy" porn aling the lines of Playboy or a souped up version of popular mens magazines. Some surfers really appreciate that kind of photography and videos that cater to high rez glamour and tease photos rather than just going for the "money" shot mentality. They can convert quite well as long as you do a good job of pre-selling your surfer correctly.
As fas as 18+ nonnude sites go I would imagine they would have to have well above average model interaction to retain though that's just a guess on my part. As cj pointed out there are plenty of sites out there that are models who just don't wanna get nude but don't mind doing the hard tease. I can see where they would convert because the model and her tease are the selling point but I would imagine the retention efforts would require more but could pay off with loyalty just like a hardcore or glamour model single girl site does.
I've joined exactly one site since i've been on the web and it was a softcore site
http://www.suicidegirls.com Sykkboy told me about it and I checked it out and it was a damn good site.