See, I think that you start out right, but I disagree with the second part. Just having the right answer isn't enough. You can come up with the right answer incorrectly and thus not have repeatable results. While I don't think that anyone should ever be graded on their process, I do believe that it is important for the instructor to see how you come to your results. A good teacher may be able to better direct the student to more reliable results if need be. This is especially important in tests where the school district may be able to spot trends that many students are having difficulty in a certain area.
With most standardized tests, they offer a few select answers to any given problem. A bright student can often surmize the answer without having to do the work, but that doesn't let the instructor know if they actually know how to do the problem correctly. Also, do we have to have an exact number? If I were to have a problem like 2+2, would 3.999 be accurate enough? As you get into machining tolerances, we probably wouldn't have any problems with a student not coming up with 4. However, if we know how the student arrived at their answer, it may help the teacher guide the student to a more precise answer.