Yeah ELP and Yes (sorry don't see it in PF at all) can be a bit pompous at times. But I've never understood the pretentious label that is tagged on prog bands. I've never heard any prog musician state they are the best guitarist(insert instrument) in the world. And for the most part, at least the artist, I've followed and read about the prog musicians are having fun playing the music they play. Aren't they just playing more diffcult pieces because the simple 4 x 4 time bored them to tears and they wanted to explore new ground. I thought experimental is a good thing but it seems to me its only good experimental if the critics like it.I like all three of the bands Tray mentioned, but I agree they can be Pretentious at times, still that is what I love about Prog in general.
I don't see the pretentious tag throw on any other genre of rock and if you are going to throw it on prog.Then use it on all rock bands that have solos or play anything even remotely difficult. Heck I've seen enough concerts to tell you that I've seen pompous muscians in every genre including Punk.