Well, that's true, but it's not the whole story, and students should rightly wonder about this when other professors are using a ten-point scale. But here's the real purpose for the seven-point grading scale: the theory department has tried to enforce a 70% knowledge requirement as a measure for passing a theory class. That means a student should show knowledge of 70% of the material in order to pass on to the next set of classes. Since University policy is that a "D" is a passing grade, the seven-point grading scale builds this in, whereas the ten-point grading scale would allow someone to move up to the next section with knowledge of only 60% of the material.
Looking at the above diagram, imagine a student — we'll call him Ludwig — who takes Theory II from me and earns a 63% for a final grade. If I am using the ten point scale, that means he gets a D. UNC considers a D a passing grade, so the registrar allows him to sign up for Theory III, getting around the theory department's hope for a C or better to move forward.
Using a seven-point scale, however, means that Ludwig's 63% translates into an F, meaning he gets to enjoy Theory II for another go-round.
Now, if the theory department were consistent about checking student grades at the beginning of each semester for every course, and dropping students who didn't have a C or higher in each prerequisite class (yes, professors can do that!), then this wouldn't be an issue. But that fact of the matter is that it hasn't been consistently applied before now.
Starting in the spring, however, it will be done, and in fact the School of Music Curriculum Committee is currently looking into the possibility of having it be a registration-based requirement (i.e., students with a D in a prerequisite class would not be allowed to register for the course in URSA).
With that being the case, a seven-point grading scale would be unfair, since someone with a 74% would not be allowed to move on to the next section.
So, as of Fall 2008, I am adopting the ten-point grading scale. I know that my students will like this, because it actually represents some grade inflation (a 90%, which used to be a B, is now an A). But there is that awful drawback: A "D" is now a failing grade in theory courses.
Incidentally, this coincides with another change, which is the addition of plus and minus grades. The university is allowing individual professors to decide whether or not to adopt this system; I plan on doing so, because I think, for example, a B+ is a better grade than a B-, and the student should deserve credit for the extra work involved.
The almost humorous effect this brings into play is this: students used to hate my grading system because a 93 was the lowest "A." Guess what: 90-92 is now an "A-." So guess what score is the lowest A?
(Cue maniacal laughter here.)