RS has been doing "500 greatest" lists probably since they started in '67...it's all just the same old crap (excluding the Fab Four, of course :D ) reshuffled every so often to show a nod, however token, to the "younger folks" who wouldn't know Simon and Garfunkel from "Simon and Simon"...and wouldn't know either anyways. 
I'd have a lot more faith in these lists if, say, rock music was no more; that is, you can't concievably come up with "rock's greatest songs" until the art form itself is dead and replaced with jazz (slight nod to KITH skit about Death of Rock...not that anyone got that).
When the final nail in the coffin of rock and roll is hammered in (my guess is it already was with "Christian Rock", a bastardization which completely missed the point of what rock was about to begin with), THEN I'll buy any list of greatest rock tunes. Until then, it's all a bunch of horseshit...to sell more magazines 