What does meringue powder do in butter cream icing? I've recently been taking a Wilton cake decorating class. Part of the curriculum includes making our own buttercream icing. Many of Wilton's recipes for buttercream and like icings call for the addition of meringue powder.  Other icing recipes that I've found online seem to be split as to the use of meringue powder. I've made many buttercreams in the past and have never used it before, and haven'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 shortening-based buttercreams that otherwise would not require refrigeration make my icing require refrigeration?
The crusting is important so that any flowers or other intricate decorations will fix their shape and not slump when left out in warmer temperatures.  (you don't want to refrigerate iced cakes, as when you remove them from the fridge, you'll get condensation).  An icing that crusts quickly means that you can use an icing that's not quite as stiff for your decorations, and after a few hours of decorating, you'll come to appreciate that minor difference.

The crust is also a benefit when trying to get a perfectly smooth cake.

Some other recipes will crust over time, and the recipe that I got from one of my instructors  uses more powdered sugar and cream of tartar, but no merangue powder.  I suspect it's the starch in the powdered sugar that ensures that it still crusts up.

As for shelf life ... I'm not 100% sure.  I believe that the eggwhites have been pasteurized,  and I've made flowers, let them dry out, then kept them stored for months at a time ... but I can't remember if those were made with merengue powder or not.  I suspect that some batches were.