Bruce is good at singing like Bruce and Paul is good at singing like Paul. Bruce singing Paul's songs on the live albums doesn't sound quite right, and if Paul ever sang Bruce's songs it'd be even worse. Although most prefer Bruce, one is just as good as the other. Had Paul not left the band, I think they would have achieved nearly the same success.
This is a trick question. The real answer is Dwight Shrute. He will fill in for Bruce on the tour for their next album, and surprisingly rock it. And the answer is yes, I did just pull something from TV into the music thread.