There is a right way and a wrong way to flash the band. Both are distracting in their own ways but generally one is more accepted than the other. Having been on the receiving end of both good and bad flashes I thought I could share some insight.
The right way to flash the band often (but not always) involves the female form and a briefly exposed pair of breasts shown as appreciation for the music. It often results in many members of the band missing a note or two but overall it increases morale for the band and that results in a much better show for the audience. Also acceptable for showing appreciation for the band is moshing, beer spitting (in some scenes) and tasteful microphone hijacking for those raucous sing along chorus.
Very important and serious side note: Obviously I do not play in the kind of mysogynistic bands that encourage women to "show us your tits" because we all believe breast viewings offered out of love and appreciation for the band are better than ones offered to shut the assholes on stage up.
We believe bands who piggishly demand boobie flashes from audience members should take a course on manners before being allowed back on stage.
The wrong way to flash the band has to do with photography and I can honestly say that getting blinded 97 times during a 30 minute set is 100% annoying. (I'm not bitter, I'm just sayin it kind of sucks) Stage lights, while hot and bright, are typically gentle enough on the eyes as long as you don't look directly at them, kinda like the sun. On the other hand, surprising bursts of bright white light eminating from the audience and directed towards the face are both distracting and painful. They often result in missed notes from the one blinded member of the band and it makes that blinded member of the band look like a hack. It also sucks for the audience members who happen to be stuck behind the flasher.
On the photography side, unless the photographer has a firm grasp on the technicalities, flash has a tendency to wash out the stage lights in your photos which are half the fun of photographing live music performances. If you have to use flash (and sometimes it is necessary, especially when photographing those elusive bass players and drummers that lurk in the shadows or to achieve a certain style ) be a considerate photographer and use those bright lights sparingly or, even better, ask permission from the band first. Remember my photographer friends, it's the band's show, not yours so try not to hog all the attention with your light show. Super bonus points if you can remember to put the camera down and enjoy the show.