ELP, Refugee, and U.K. (second and third albums) are all keyboard-based prog rock, and they definitely rock. Guitar does show up in ELP, but Lake mostly plays bass with that band.
I agree with whoever said that drums are required to rock, but pretty much anything else goes. One of the cooler bar bands I've seen was a two-man band, with one guy on drums and vocals, the other on keyboards and vocals. All areas were covered, and they even had some vocal harmonies.