The trouble with your logic, Hurlshot, is that it only really applies to grade systems with a small number of possibilities.
In reality, the systems used tend to be out of 20, 50 or 100, not 10. This allows for more detailed differentiation of somebody's work without causing them to be disheartened.
This is further evident in Pixies' sarcasm above, where it would be ludicrous to only rarely assign an A.
You again ask "why have the ten available at all?" but I answered that earlier, so I won't repeat it here.
Oh, and if you feel that kids should be assigned 100% from time to time just to keep their hopes up, I'm afraid I strongly disagree. At some point in their schooling, probably mid high-school, they definitely need to learn that their work is rarely perfect and that there's always room for improvement. Because at university and in employment this will certainly be the case. Where I come from, 100% scores were rare as hell, but that only made us try harder, and it made a high score like 90 seem all the more noteworthy. I appreciate your attempts to think from the mind of a student in this regard, but I think your eventual conclusion is flawed.