Deke Sharon, the All-Father of A Cappella, hits it square-on with his latest rant/sermon/moment of inspiration.
His premise: many people would readily agree that they are fine shooting hoops casually in their driveway, without aspiring to be NBA players. Similarly, a little casual singing in the shower is different than practicing and honing one's skills to be an accomplished performer. Both are good - both are communication, and should be part of the fabric of our lives.
"Encourage your friends to sing. Expect them to not be amazing right away." Be tolerant.
(Deke's name comes up often on this blog -- because the man is right!)