I am not condoning the use of hard drugs but when it comes to certain musicians it does help them get through a performance. Most of them are either addicts or have some sort of condition that prevents them from performing sober. As long as it does not hurt anyone there shouldn't be a problem.
If every musician, a band was sober then the music wouldn't have been so great, whichever side of the fence you're on, psychedelics and hooch were mind opening things no matter how 'dangerous' they were to be. No mention of Jim Morrison in Miami or in New Orleans? Syd Barrett's infamous TV moments ? Janice Joplin on stage feeling no pain?
Yes, reason stands that some performers could do their job better while on drugs. Musicians living on buses and performing every night during a tour get very little sleep or time for themselves. It is logical that they would be exhausted and be unable to perform their best every single night. Cocaine is supposed to make the user more awake, so it seems to follow that an over-worked musician would give a better performance while using this drug.
Not that I condone drug use, but I can certainly see where it could help a performer through a show. Being on stage is an exciting, nerve-racking experience, and having drugs in your system that essentially change your mood and personality, a performer is able to get through a show with energy and an outgoing exuberance.
If the music is complicated like say Dragonforce then no they don't and dragonforce proved that with their terrible live performances before they stopped getting drunk and what not before performing.
If the music is more simple, 4 chord stuff like pop-punk (simple plan, sum 41) then I doubt it'll make a difference. I've performed pop-punk stuff not sober before and it was fine but I've played Metal not sober at home and it was awful.
Depends how experienced of a player they are too. If you've been playing your songs for 20 years in rehearsals and at gigs then sober or slaughtered you'll be fine.