", ...I believe there is a completely valid place for those with a lower level of natural ability."

*grin*...*I* have that. But I 'can' do a credible job on some things, and if I practice, I can pass in social situations just fine...though you'd never pay to hear me.
There's quite a difference between "... a lower level..." and rock-bottom. It is often difficult to tell whether someone like Bruce O. could have been helped with lessons, but we'll never know about HIM.
I do know several who have made serious improvement over several years...they KNEW they needed help, and they got it...and they practiced.
It's a fuzzy line to draw in deciding whether you should or should not sing & play in front of others, but while the gray areas are hard to define, we all recognize the extremes.