Any performer should be at their best when performing. People pay money to be able to see a performance, therefore, they should be getting their money's worth. If a singer goes on stage inebriated, the cannot possibly be performing at their best, or maybe even not performing well at all. They should refund the audience's ticket price.
Singers should perform sober to give audiences the best show they can. Also, public intoxication is technically illegal in many municipalities. Therefore, performers that are drunk would actually be breaking the law in many venues. Furthermore, most singers charge a high price for tickets; so performers should give concert-goers their money's worth. In short, no, singers should not be inebriated when they perform.
Singers should not be inebriated when they perform because, without question, inebriation does nothing but hurt and diminish the performance abilities of the singer. While having a few drinks may help the singer to overcome nerves or stage-fright, in the end it is just a bad idea. Conversely, some singers are so bad that maybe the crowd should be inebriated instead.
Whether it's an intimate venue or a huge stadium, singers shouldn't be drunk when they perform. If I spend $200 for a ticket, I want to see and hear the best of the artist. A sloppy slurry performance at a premium price is beyond disappointing. Feel free to get drunk later, but not when you're performing your show.