It is too difficult and time consuming to change lenses in the darknes and crowd of concerts.
I do that all the time. Of course, if you're in the middle of the crowd on a rock concert it might be tough. But then taking photos would be tough anyway.
A thing that has worked for me is to look serious and dedicated, and let people see that you've got a bigcamera that looks expensive.A camera bag to go with it helps even further. Then both crowd and crew give way, since they think you're a pro.