A problem that might occur (in kyu system) is that students start to dictate what is being practised in the dojo. How so? Different students train for different grades. Everybody requires time to practise their set of techniques in order to prepare for their test.
There is limited time to the test so the teacher must somehow fit all the techniques in the remaining lessons. Next iteration the students want to train for the next grade and again the teacher must somehow fit in that set of techniques. The teacher is no longer in control...
A teacher who can't control his own curriculum has bigger problems.
Especially for black belt and higher level kyu ranks, this problem is generally solved through an expectation that the student will spend time on test prep outside of class.