I've recently been taking a Wilton cake decorating class, part of the curriculum includes making our own butter cream icing. Many of Wilton's recipes for butter cream and like icings call for the addition of meringue powered. Other icing recipes that I've found online seem to be split as to the use of meringue. I've made many butter creams in the past and have never used it before, and have't been aware of any specific shortcomings.

What is it that meringue powder does to the icing? I've heard that it will cause the icing to "crust", how is this advantageous, and what am I missing by not using it?

Also, given that meringue powder is essentially egg whites, will using it in shorting based butter creams that otherwise would not require refrigeration make my icing require refrigeration?