This is, of course, a matter of personal opinion. Moreover, it is (at least in my view) a very difficult question because there are compelling arguments to be made both for and against discretion.
On the one hand, it seems that it is very important to allow these parts of the criminal justice system to exercise discretion. If we do not allow them to do so, we risk creating situations in which unjust outcomes occur. This is particularly true because of the harsh sentencing laws today. Without discretion for judges and others, we can have situations where people go to jail for long periods of time for crimes that do not seem to warrant such harsh punishment.
On the other hand, discretion can be a dangerous thing. It can easily lend itself to abuse. A judge or a prosecutor of one race might be inclined to use their discretion to help members of their own race only. This may not even be conscious, but it would be unjust. Discretion could lead to more conscious abuse as well, with prosecutors or judges making the best deals for the clients of lawyers who are their friends, for example.
Thus, we have a real quandary here. We risk unjust outcomes whichever way we turn.