Certain voices , although very good, don't lend themselves to good BU vox. They are too distinctive. Like Lou Reed e.g. Great lead vox but not enough of an intrument to do good BUvox. There is a certain anonym,ity that BUvox need. Especially if you're stacking a whole choir. In his day Keith did superb BU vox, but they were their own kind because they were like special; almost a second lead on top Jagger. bernard is good because he has no instantly recognizable charcteristic. Jagger in "You're So Vain" is part of the choir but immediately takes over. Same goes for "Buried Alive" "She's Just 14" on Pay Pack and Follow.