Agreed with Brains. Bo Rhap kicked ass. As for essential band members:
Beatles: all 4
ABBA: all 4
Police: all 3
Queen: might get away with no John Deacon, but it's touch and go.
ELO: Jeff Lynne, certainly, others I'm not 100%
Foreigner: Mick Jones & Lou Gramm (the only members still in the band now)
Sytx: Dennis DeYoung, Tommy Shaw, not sure about the others.
The difference between genius and insanity is merely the difference between success and failure.
Led Zeppelin: I think must have 4 of them, J.Bonham and JP Jones are essential, though many people say that Zep is the duo Plant/Page. So much so when Bonham died, Zep broked up.