the domain is not all that great - you would be buying the 404 traffic... (which will convert well no doubt), but it is limited.. buying a going concern would be a much better prospect..
If the free hosted sites were given a subdomain, it would make it more difficult to profit from buying the name alone... if the sites were on
www.freeadult100.com/sitename - that would make it much easier, a small trick with htaccess and a well placed server would help get good SE traffic.
Of course buying the service up and running would be the best option, and if run by the right person, it could make serious bank.
I am planning free CJ hosting soon... if you are interested, just ask! I am sickof all these servers gathering dust...lol ;)