The Yardbirds were hugely influenced by and respectful of the blues masters
who preceded them.
It is too bad that they were forced to lip synch for that show but I'm sure many of you professionals can relate to the restrictions you sometimes have no choice about.
Did Keith Relf do a crappy
job of it that night. Yeah, pretty much. But he (and the Ybirds) brought
music with strong roots into Britain and the USA at a time when most other
bands were not doing that.
Sonny Boy Williamson respected them enough to play with them. And they
idolized him.
http://www.cduniverse.com/search/xx/music/pid/1001203/a/Sonny+Boy +Williamson+&+The+Yardbirds.htm
Keith Relf was hugely talented. He died at age 33 and is not here to defend
himself.
You can judge him on one crappy performance if you wish. It's not representative of his work.