The type of night, audience expectations,aims of the performer, combined with subjective views on what is "poor" make this a tricky area.
My personal view
1. club nights where there is no or little charge and people usually do two/three songs "poor" performances make little difference.
2. guest nights where there is a more significant charge then care needs to be taken to ensure any support spots are of a "reasonable" standard, the occasional poor performance will probably be excused if it is relatively short.
3. major concert - get it right both in terms of style and skill of any supports as the audience is usually more varied and has paid good money to be entertained.
Booking anyone is subjective. Professionals usually have recordings,videos reviews etc which can help, others who are semi-professional or amateur can be more of a problem.
I play in a trio, we see this as a hobby and are not chasing money we get club and festival spots usually from people who have seen us play. we take the view that with us what you see is what you get we have a few live recordings which together with actually seeing someone are the best way to decide wether you want to book an act.
amelia earhart's last flight
i guess for many of us,particularly in the folk world where many of the audience are also performers, the judgement of "poor" versus "worth a short spot"versus "worth booking for a full concert", is derived from how we view our own performance standards, and any particular type of music we favour.
Anyone who really wants to sing/play will be able to find a niche it may not meet their aspirations but hey its music.Enjoy.
John