Well, my quarrel was with the thread title, not the question of what makes for excellence as a vocal performer.   
As for how good you have to be before you should get up and sing before an audience, I still hate to discourage people from doing that until they've achieved some standard of excellence, because most singers have to sing somewhat badly in public before they get the confidence and control to sing really well in front of an audience.   And I think that goes both for vocal technique and things like connecting with the audience, conveying what the song is about, etc.
Personally, I agree with Ian and Richard.   Some of the singers I've enjoyed most and who've expressed songs best have been rather mediocre in terms of pure vocal talent & technical skill.