Students love improvisation and for that very reason, the faculty must discount improvisation as the domain of the unrehearsed, the imperfect and the consequential shortcut.
The reason students love the idea of improvisation is because they believe it is unstructured and they can make it up as they go along.
That is why improvisation is dangerous in its core to learning: There are no rules, foundation or context to help the students learn something memeingful and precise.
Just like anyone who can construct a grocery list believes they are an author; so too, is the misguided student misled to believe improvisation has any meaning beyond the self.
Student improvisation tends to be predictable, unshaped and imitative. Improvisation does not serve or require creativity or imagination and that is a bad thing.
If the instructor is looking for an easy out in the teaching — then let the students improvise at will. Everyone will look busy even though nothing is being taught or caught.
Inspiring the imaginative student — by providing strict rules and conditions for the assignment — is precisely the role of the rightful instructor and the only path we must tread if we hope to remove our children from the busywork morass of attempted and failed mind conditioning.