The funny thing is that back in the 70's, many music critics often spoke of Led Zeppelin in the same breath as ELP and King Crimson. When prog went out of fashion, they bashed for the same reasons are beloved prog bands were-being over intellectual and such. The fact they are blues based shouldn't be held against them either; prog is about combining different genres rather than excluding them.
Then again, in retrospect, Led Zeppelin aren't really prog by todays standards, and prog wasn't really ever used as a label back then. When we look at them today they're strait up blues rock with some prog influences and cool jams.
Led Zeppelin are possibly my favorite band ever. I'm not sure whether or not they should be included, some people wan't them included just becuase they wan't to think every good band is a prog band.