When you shot low light and very high ISO the images become a bit blurry and grainy, but bear in mind that no one looks at photos in a hundred percent view. The contents of the photo are important, not the quality. This is not fine art photography! The photo above was shot with wide open eyes at f 2.0 to add some depth from the crowd to the stage. The light from the right creates a great mood and the audience becomes a part of the whole thing showing how dense this place was. I also love the hat of the guy in the right on stage. It is a visual anchor in the whole image.
Motion blur is also great to create some movement and emotion. It is not always a sign of bad photogrpahy. This guy was touched by the music clapping his hands.
In my conclusion a fixed lens camera like a X100s is a good companion for concerts. You have to move and you can't do the same stuff like with a telephoto lens, but if you try to do more reportage style photos the results can reflect the emotions at and around the stage. The limiting factor of this camera can be a huge advantage.
"We believe in limited resources, don't we? Absolutely. I'm dead keen on limiting resources." - (Delia Derbyshire)