In reality, most diplomats are representatives of nations with a tradition of professional civil service, and are expected to obey regulations governing their behaviour and they suffer strict internal consequences (disciplinary action) if they flout local laws. In many nations a professional diplomat's career may be compromised if they (or even members of their family) disobey the local authorities or cause serious embarrassment, and such cases are, at any rate, a violation of the spirit of the Vienna Conventions.

http://en.wikipedia.org/wiki/Diplomatic_immunity

It's not like they get a blank check to commit any crime they want, and there are good reasons why we have diplomatic immunity.