You seem to be presuming that a curve is lowering grades to distribute them, when the only time I was graded on a curve, it was an increase to distribute them.
My definition of "graded on a curve" is that the cutoffs are set after the test is given, based on the performance of the students on the exam so as to (approximately) get a desired grade distribution. For all the reasons that other people have said, I think this is a shitty way to grade.
I'm not arguing hard tests are bad*, but the instructor should set the test based on what portion of the material is worthy of each grade. In your case, maybe the instructor says "hey, I know this is a hard exam, so 60% is going to be a C". (They don't necessarily need to tell the class this, just know it and try damn hard to stick to it.) The important thing is that the scales are set by the instructor based on how much mastery of the material he or she feels is needed to hit that level.
This isn't a hard line in reality -- for example the scores could come back and the instructor goes "damn, that was a lot easier/harder than I thought it was going to be" and maybe adjusts things a bit, or maybe they plan 65% to be the cutoff between two grades but then there's a cluster of scores right at 65% so they bump it a little to one side or the other -- but I think the latter is definitely an ideal to strive for. And that's true even if the instructor sets their expectations and hence the scores based on the grade distribution they want -- it's still a different mindset, and I think that both that mindset is important as well as not making your students compete against one another directly.
(* I do think that working hard to stratify students is probably counterproductive to what is supposed to be the goal of education, which is to teach. But let's not go there for this argument.)