What does meringue powder do in butter cream icing?
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.