"Isn't the intention to glorify God in itself glorifying?"

Perhaps. But that does not change the fact that while the critical judgment of all art is ultimately subjective in nature there are also objective standards that can be applied to any art form. I'm of the opinion that music that is performed poorly because it was not rehearsed enough (due to laziness) or performed by people who weren't up to the task in the first place, does not necessarily glorify God, at least in any way that can be recognized by the human ear. That may have been the original intent, but the overall effect is often the exact opposite, at least to my ears.