...people who don't have decent performing skills should stay home until they achieve them...

But you don't know if you have performing skills unless you get up in front of an audience. Then, when you find out that they are not all that you would desire, the only way to hone those skills is in front of an audience.

No amount of practicing at home is going to prepare you for being on stage. At home you are just practicing your mistakes.

Any venue that doesn't cater for and encourage at least a few beginners, is doing a great disservice to the singing and listening community.

You have a choice of two options if you start singing and find that you are out of tune. Both take a courage that can only be learned on stage.

1. Stop, say 'oops' and do a quick relative tune on stage.
or
2. Put down the guitar and sing A Capella.