Thanks. I had a professor
(Professor Emeritus now, BTW), who mentioned Deep Purple. He said they were successfull because they could actually read music.
Listen to "Fascinating Rhythm" from Gershwin
http://www.youtube.com/watch?v=iLQtMEh7lX4
Now listen to "Burn" from their last album with Ritchie, before Tommy Bolin joined.
http://www.youtube.com/watch?v=LCnebZnysmI
They were all competent and talented musicians and they defined the genre of heavy rock based around a guitar/keyboard lead instrument and showcasing a player's abilities.