someone on gfy sells a porn cms? cool!
now to answer the actual fucking question (without just trying to sell my shit), yes most sites are moving to prebuilt software. prebuilt software has better features and more usability testing.
5-10 years ago there just wasn't any quality adult cms to choose from. now there's a least a few good options. each has pros and cons. depends on your needs. i'd make sure a paysite cms is easily customizable (front and backend) because EVERYONE in this business has their own quirky way of doing business. you can usually get some references too.
oh, and just to point it out, my software is by far the best. fuck the rest.
http://www.porncms.com/
(adult interface is pretty cool too. everything else is at least 2-3 years behind in technology.)