Same can actually be said for Casual RPs, Ruby. 

With the right group, be it a casual or advanced group, both playing and reading the game can be very interesting. It could probably be said for free too, but we don't have any good experience with that, so we can't say for sure.

The "purity" of an RP doesn't hinge on the post length or even perfection of the language. It hinges on how well the players mesh. If the players work well together and can adapt to each other easily, then the RP will usually end up being an enjoyable story, no matter about the factors like language, post length or such.

Some players are more suited to advanced, while others prefer casual. And of course there's those that like them both.