I've never heard of Emerson, Lake & Palmer and I'm sure many others haven't either. They are probably underrated. So many musicians and groups go unnoticed because they are not at the center of controversy like many popular artists today. It seems you have to have some negative attention in order to be popular in today's society.
Yes, Emerson, Lake & Palmer is underappreciated as a force in the music world. This band put forth at least one very successful tour, one show of which is now heavily bootlegged on the internet. All students of aural media should appreciate Emerson, Lake & Palmer for the contributions they have made.
Emerson, Lake & Palmer is not one of the most common bands. Rather, a lot of people don't know their work. Too often, a few bands become very famous while so many other good bands are completely unheard of. This is because it's easy to get music to the entire world, whereas instead of just going locally to hear a band.
The heyday of prog rock is a distant memory, even to fans of the genre. With the exception of Pink Floyd, many of the bigger acts of the day have been somewhat forgotten over the years. The fans still remain strong, but styles have changed to a general audience, and you can't expect everyone to appreciate a dated genre.