2 stories. Firstly, back in the 90's a very highly regarded Female singer (I won't name her here) doing a very high profile concert, noticed someone filming her (without permission) on a camcorder. She stopped mid song, and without explanation, walked off stage. She only came back, after the bloke had been ejected. Quite right too!
The other occasion, which turned out to be Ivor Cutlers last performance. He stopped mid song, and said..."Hey, You in the front row. Are you recording me?....(pause).....Don't"
Most big rock gigs I've worked on, photographers get a very strictly controlled amount of time in the safety area in front of the "Mosh pit" to do their snapping. and are then told to leave. This normally happens in the first couple of songs. (As people have said, a photo can't tell you anything about the song!) After that, the audience can enjoy the gig.
At a Folk gig (Pub/Coffee House, whatever) It's just simple courtesy to ask first, as a kick in the teeth often offends. Most artists I know would be only too happy to agree, (as long as it was done discretely), but, would always insist on being sent copies. (pretty simple nowadays.) You never know, the photographer might just have captured that magic moment that becomes the cover of your next CD!
So, Restrained photos with the artists agreement, Fine...Standing in front of the stage with a large lens camera, shooting up the singers nose, and pissing off the audience...No!