He was supposed to sing it on the album; it was composed for his voice. Harper only sung the song because Roger couldn't do it at that point.

About singing live... It's not just the fact that you only get one chance, but so many other things. The tension, the lack of 100% accurate monitors, the noise people are making, the tension, the EXTREME heat on stage, the spotlights burning your eyes out, etc. The worst part is, in my opinion, that you're not able to hear your own voice directly; you hear it through the speakers in front of you. Which makes it very hard to pitch correctly, since it's really unnatural, as if you're listening to somebody else.