First of all, police are simply not trained as rigorously as soldiers are. Cops have routinely been seen, in public, brandishing weapons in ways that no soldier would ever do: it's a matter of basic gun safety, let alone threat escalation.
Second of all, military equipment is rarely designed for the kind of tasks that the police have to do.MRAPs are simply too big, unwieldy, and end up probably being more expensive than a Bearcat, and most counties simply do not need it.
Third, the only people who we can be sure benefit from the militarization of police are arms contractors. As always, corporations and their needs are first and foremost, and everything else is incidental.
Most importantly, the militarization of the police is a symptom of a deeper problem. The fact that the police too often view themselves as being in effect the occupying army in control of a city, rather than civil servants and fellow citizens keeping the peace and being helpful, isn't just one of the major causes of police brutality but also one of the major reasons why crime control so often fails in America.
Right now, SWAT teams and military hardware are just overused. Simple warrants for non-violent drug crimes should not require a SWAT team or assault rifles.