Tea with the Queen, parchment documents of historical import, and speeches are but three examples of where formal language is used to make clear the infeasibility of unconstrained responses.
Similarly, to employ formal speech in situations of some intimacy is to court pretension, as I do here. Furthermore, should formal speech be surrounded by more relaxed dialect, the contrast can cause an exaggeration in the perceived level of formality, y'know?
I contend that - because of the easy facility with which one's electronic audience can engage in discourse with email authors - excessive formality in email is inappropriate. Would one encourage an image of a soliloquizing bureaucrat, disinterested in dialogue with the teeming Internet community? Or would one rather present a portrait of a kinder, gentler entity, one that has a genuine interest in providing answers and receiving feedback?