To put it another way: would students learn better/faster in school, if every assignment was designed to be impossible to complete? (Obviously, the assignments would thus go un-graded, but frequent micro-examinations would be inserted to assert the learning derived from all the failure.) As an interesting side-effect, in such a curriculum, any student that succeeded on an assignment could be immediately skipped ahead to the stuff they can't do.
The point is that it's good to reach beyond your grasp every once in a while. And when you're trying, you can't always tell if the thing you're grasping for is too far away. You just reach.
Doesn't sound like fun for the crowd or the teacher...