I have heard some "bad" singers in my day who eventually got pretty good, or at least passable, with time. Thing is, it may not take the same amount of time for some as for others. How long are we willing to wait?

Everyone deserves a place to sing. And if a club or cirlce is open to the public, ya can;t throw people out for being lousy.

As for clubs hiring "bad" singers; that is their prerogative. One can leave, go drink in the next room, or sit quietly and wait for a better singer to come on.

It might be worth encouraging "bad" singers with advice or suggestions of material. Let them know you think that )insert title here) would be a good song for them. Maybe it's in an easier melodic range than what they normally sing. Maybe it's not as rhythmically or lyrically challenging as what they've tried in the past. (This can backfire, of course--especially if the singer has been led to think they really are worth being paid for what they do, or are hired/booked repeatedly).

I am not a fan of mediocrity. But no one should be denied the right of making music with others.