It's important that more people see officers and get the impression that they are there to enforce peace, not dole out violence -- while still ensuring they have the tools to maintain safety and order. Large, threatening military-style vehicles don't send a good message to people who are already scared for their safety.
I don't remember who said it, in relation to sports - "Look good, feel good, play good." I think how you look can absolutely affect how you behave.