On CD isn't the issue. Virtually anyone sounds good in a studio setting. It is listening to these same singers live, Robert Plant included, that is the big let down. What singers sound as good, live, as their studio work? Orbison, Franklin, they impressed me live. Rock vocalist in general are a big let down on stage.
Annie Lennox, Pat Benatar, Lita Ford, Bangles, Cindy Lauper, Belinda Carlyle, Celine Dion, Mariah Carey, Def Leppard, Europe, Great White, Bon Jovi, Survivor(Eye of the Tiger), Kenny Logins, Robin Zander(In this Country(Over The Top(Movie), Journey(Steve Perry), Foreigner, Scorpions, Van Halen, David Coverdale(Whitesnake)Chicago, Starship, Air Supply, Cars, The Who, Wham, Firehouse, Poison, Bryan Adams, ALL had FREAKING AWESOME SINGING VOICES, as ROCK singers.
But that said, your right, that they are the exceptions, as most of the ROCK singers are not that good.