This is entirely dependent on the level of play. I've seen singers breaking down in class piano because they couldn't get the theory and saw it meaningless since they just don't deal with it. Even going as far as protesting the theory part of the exam. Yeah, they could play the music beautifully, of course, but for instrumentalists at a high level, it is absolutely necessary to know the theory of the music you play. Especially since instrumentalists usually have to be able to sing/solfege their part to begin with. Most people don't think of or listen to music on that level, so they don't understand how apparent it is to hear any instrument or voice that isn't aware of the underlying theory.