Good is subjective like you said. You can have RP's that are written well, you can have RP's that are fun. Either counts as good if that is what the participants are into, and if an RP is both of those things I suppose it's pretty good then.

As for if it is an art... I think that depends on what you mean by "Art". I'd contend every rp is art in the sense they are all creative works. But if you mean art as in "Good art", then that's harder to tell. I can't say I've seen casual or free rp's that are organized enough to qualify, but I have seen some pretty decent advanced rp's. The thing is, this forum has plenty of decent writers. If they get organized to write a coherent plot, I think it can absolutely be considered good art. Hell, I'd go as far to say the only thing between a lot of the advanced writers here and publication is the limits of the literature market. 

As for the question of how it compares to other forms of art... no, I don't think RPing is high art if that is what your thinking, but that shouldn't be discouraging. All high art is is the stuff people have decided stand out amongst all the other creative achievements. I don't see anybody grabbing an RP and deciding it needs to be taught in an english class anytime soon.

As for the free/casual/advance split, because I mentioned this earlier and you mentioned it, here is my thoughts. It isn't impossible to imagine a free or casual rp becoming organized enough to be compared to a script, i've just never seen it happen. Advanced has the upper hand in the artistic merit debate since advanced approaches literature more closely and the slowness makes it easier for something thorough and thought out to come together. Not to mentioned advanced RPers, because of the requirements, only attract people willing to make the effort, which makes it easier for the good writers to coalesce.