The curve helps control certain problems with test. A problem that was really just too damn hard for almost all the class. For instance a problem where all but one or two students in a high level class can come to grips with in a half hour or so. Material the teacher meant to cover, but didn't. You can say it's all the professors fault and that they should have written a better test, but many professors got where they are by excelling in their field, not by being fantastic test writers.

I would rather have a percentage of professors that are respected experts in their field of study then just professors that are good at writing comprehensive exams. Most students look at these things only from a fairness perspective and don't value having talented practitioners with a few teaching deficits, but experts are good for university programs overall.

Not that the curve is the best thing ever. I remember an mathematical econ class where there was a student who consistently completed exams in a half hour of a two hour exam who got every problem correct, with no other student scoring 100%. That guy really harshed on our curve.