To say ELP is one the worst bands is flat out BS. They can play circles around most anybody. To say Insane Clown Posse is even music is generous. Besides I thought The Velvet Underground was the worst Rock Band.
No one doubts the musical skills of ELP. Great musicians, all. It was their somewhat bombastic amalgamation of rock and classical that warrants their inclusion, IMO.
Worst band ever? Got to be Power Station, the "supergroup" of the 80s with that Duran Duran guy, Robert Palmer and some other people.