Mandatory sentencing removes the ability of a judge to rule based on the individual and the record of the individual. It is important to allow some thought in the process as a person with no criminal history should be sentenced differently then a person who has previous criminal history. It would also take away the ability to allow entry in reforming programs or services to be used in place of fines and jail time.
The punishment should fit the crime. It's improper to put individuals away for simple infractions of the law. It doesn't leave an room open to interpretation for judges and juries. If all forms of murder carried the same penalty, would it be right for someone who accidentally killed someone in an auto accident to get the same sentence as someone who kills their family with a chainsaw?