Some claim that if you can't dance to it, it isn't rock and roll. Well, let's not go through the Beatles catalog, although that would be the shortest way to refute this contention. Let's see a show of hands. That aside, rock is a very inclusive idiom, where some music makes me want to think, and other music makes me want to dance, and once in awhile does both. The important thing with any music is that it makes you feel, whether it resonates with your emotions by making you move or making you listen.
Progressive rock resonates with the emotions very well.
Some people say that progressive rock artists haven't influenced anyone other than progressive rock artists. Although this last one does address the issue of influence, it's a circular argument. It assumes that progressive rock artists don't count, so influencing progressive rock artists doesn't count. What I find even worse is that artists who have been influenced by progressive rock or who have explored elements of progressive rock, with the lone exception of The Beatles, are then labeled progressive rock artists, despite everything else that they've done. Using this logic, one could say Bob Marley only influenced reggae artists, and that Eric Clapton and The Police were reggae artists.