I am not a native English speaker, I am a linguist (Russian), but I really think that all language teachers have to be aware of one or two linguistic theories at least. The history of linguistics, for example, is obligatory for all linguists. It's even a must-have! A teacher, who knows linguistic theories, is simply more professional or at least seems to be more professional. I know that teachers and doctors are the two professions that require natural-born talents but still it is much better to have some linguistic background rather than to be just a "practising teacher". Plus a classroom may become your laboratory in a way (as long as it doesn' harm your students, of course
).