In fufilling his request, we are virtually at war with ourself. We are all too painfully aware that we are not "great" singers, and this snapshot view leads us to be silent when we should be joining the choir. After all, we wouldn't want our failings to tarnish the success of others.
It's not that different from the challenge in trying to be a "great" person; we seem destined to fall short, so why try at all? Wouldn't our sorry example just end up discouraging those who need all the encouragement they can get?
Yet, when enough voices join in, an interesting phenomenon occurs; the less-than-angelic voices (like mine) not only get drowned out, they seem to actually add to the overall value of the experience. The individual notes may contain our failings, but they nonetheless also project our act of faith. We no longer hear the sour notes, the building instead echoes with the sound of earnest effort spent towards fulfilling a worthwhile mission. The louder the volume, the more likely the reluctant singer will add his voice to the choir... a reminder that the more that people try to follow the path of a good life, the more encouraged others will be to take their own small steps along the same trail.
When it comes to being good, we're all amateurs... which is one of the reasons I get a huge kick out of this witty video: "What if worship was like an NBA game?"
[Hat Tip to James at Sanctus]