Basically, this sentencing means that the judge cannot reduce the sentence. Well, often judges will suspend a sentence to show mercy. O.K., so, do you feel mandatory sentencing is too harsh?
If you don't have this kind of laws set up, people will simply do anyhow, and break laws anyhow, and crime rate will simply be high. In order to make sure everything stays in-line, orders have to be given from the judiciaries.
I think minimum sentences should depend on severity of the crime. For instance, Virginia has a minimum 6-month sentence for "doing anything" to a cop, even just a little slap. Anyway, for people who are obviously mentally ill in a treatable way, they have to get waivers to avoid the minimum.
Mandatory minimum sentences are fixed by lawmakers after a lot of deliberation and discussion. There has to be some yard stick of measurement for everything and punishment is no exception.
Mandatory sentencing is a good law which makes sure that a minimum sentence is pronounced for any crime which will deter potential law breakers. At the same time, I think the severity of case should be considered before passing the judgement. Of course, laws exist to waive off punishment to mentally ill and such other special cases which the honorable judge will use at his discretion.