There is absolutely no excuse to not clap. You acknowledge the performers no matter what. I do not care if you liked the show, hated it, or it made you want to throw up, you acknowledge the performers because it is the professional, moral, and just flat out the appropriate thing to do. To not do so is a slap in the face of both the performers on the field, as well as the entire concept and tradition of performing arts. Those that do not clap should be absolutely ashamed. And yes, it is that important of a gesture.
*edited for spelling*