Obviously you don’t want to be a Language Arts teacher, otherwise you would have proofread. On to your question. It’s not called knowledgeable anymore, it’s refered to in the federal No Child Left Behind Law as “Highly Qualified”. If you are going to teach it, you need to know what it is you are trying to teach. If you are going to be knowledgeable, then you need to apply yourself as best as possible. I want my kids to learn from the teacher that got A’s. Think of it this way–would you want a Doctor to operate on you, that got C’s all the way through college? I don’t think so. Just because s/he tried real hard, and passed, doesn’t mean that’s the person I want cutting me open. I want to recover and return to my life the way it was prior to the surgery. That’s why we always get a second opinion. 🙂 The best doctors are called the best doctors because they are knowledgeable in their field. Shouldn’t teachers be the knowledgeable and be the best in their field? The most knowledgeable teachers are usually the one’s who are fair, more thorough in their explanation, and know how to address your learning style, wouldn’t you agree?

Are you asking, Why is knowledgeability a good characteristic for a teacher? Easy, because you are the expert in the field that you will teach (curriculum) and in addition, you must be an expert in teaching the subject (Instruction).