I'm a little confused as to what type of essay this could be turned into. I guess it could be informative, but you really don't have an issue to argue. Most states require teachers to teach in their area of certification. Certification is not necessarily their area of expertise. Some states provide certification if teachers can answer enough questions right on the state test. For example a physical education/health teacher who has never taken library media classes, but passes the certification test and is then certified to be the library media specialist. I guess this could be a situation you could argue.
I agree with the above posters. While training in your focus area is essential, cross-over can be expected. For example, California does not yet have a theatre arts credential. Teacher who aim to teach theatre must earn an English credential. Thus, they may have majored in theatre in college, but their career specific training is in another subject. Of course, analysis of plays is vital to a successful theatre program, so the English training comes in handy.
Those teachers who work in public schools are required to remain within their majors, so any discussion otherwise is a mute point. However, in the private schools teachers can be asked to teach outside their areas. Often there are science teachers who are very competent in math since so much of their coursework involves math. Certainly, a computer science teacher is competent in math as the major in college requires almost as much math as a math major. Many a literature teacher has an excellent background in humanities since the understanding of literary works is often dependent upon a knowledge of the historical context. And, many a history teacher is versed in literature since works such as The Jungle, Uncle Tom's Cabin, Animal Farm, Brave New World, The Red and the Black, War and Peace are read in social studies/history courses.
So, if the coursework is not too advanced, such as in middle school or ninth and tenth grade, sometimes a teacher who instructs out of field instructs quite well, bringing a fresh approach--at least, he/she may have more enthusiasm toward the subject matter since it is a new experience. Indeed, those who have graduated from liberal arts colleges often are well educated in several areas since their schools have stressed a broader education.
I agree with the statement in general terms; however, I do think it's easier for teachers to kind of cross over into related fields than some other kinds of jobs. For example, if I have an English degree (which has prepared me to read, write, research, and speak in front of a class), teaching a speech class is not that far out of my area of expertise. With some study and preparation, I could do the job. A podiatrist (foot doctor), on the other hand, though he has a degree in medicine, would not be able to shift so easily into, let's say, oncology--the study and treatment of cancer.
In short, then, if it's a related field there is not much lost; if there is too little similarity between what one has trained for and another field, it's not a good idea. For the benefit of all--including the teacher--I certainly would rather have an educator teaching in the area for which he or she has prepared and trained.
In my opinion, it's pretty hard to argue against this idea.
If a person has trained in a particular area, they will have a much better understanding of that subject matter. When they do, they will be able to teach it much more effectively.
I think about how much better a teacher I became after I had taught various classes a few times and I knew the subject matter much better. I could respond to questions better, I could create better analogies to help students understand the topics. When I taught things that I had little training in, I felt like I had only a superficial knowledge of the subject and could not teach nearly as well.