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.
As there are multiple types of meringues, I imagine the core of this question is really the effect of acid on egg white foaming.
According to KATERYNA LOMAKINA and KAMILA MÍKOVÁ writing in the Czeck Journal of Food Science, there is a moderately complex relationship between pH of the egg whites, and its foaming capacity (overrun) and the stability of the foam:
By the addition of a small amount
of 1N H2 SO4 or NaOH to the liquid egg white (pH values: 9.5, 8.6,
6.3, 4.7,
3.1, 1.0), NAKAMURA and SATO (1964b) obtained a great foaming capacity at the neutral and acidic pHs except at the exceedingly
acidic pH (pH 1.0). The foam stability was high at pH 8.6, the pH of
the natural egg white, and decreased with changing pH.
With an aqueous
egg albumen solution, HAMMERSHØJ and LARSEN (1999) established that
the foam overrun was the highest at pH 4.8 and the lowest at pH 10.7.
The foam stability against drainage was the best at pH 7.0 after 30
min, but on a long-term scale the foam at pH 4.8 was the most
resistant to drainage. This is the result of the more rigid behaviour
of the surface at pH 4.8 and the formation of small bubbles,
therefore a slow drainage of liquid from the foam, lower dynamic
surface tension causing the high overrun.
The conclusion is that you will get the most volume of foam at moderately acidic pH around 4.8, but the best stability of the foam once created at the alkaline pH of egg whites.
The moral seems to be that for stable egg foam, not to add acid.
Later in the article, they also indicate that volume can be increased without ill effects by adding up to 40% more water to the egg whites.
Also, adding sugar to the egg whites inhibits foaming, requiring more agitation to form the foam.
Best Answer
Acids allow more air to be beaten into a meringue. In order to make meringue, the proteins in egg white must be denatured. In their natural state, the proteins are curled up into tightly packed balls. When the egg is beaten, they uncoil into long strands.
These strands then begin to coagulate, or join together, with the help of the sugar you add. The air you whisk in gets trapped between these joining strands, giving the meringue its characteristic light texture.
Acid delays coagulation, which means that there is more time for air to get trapped in amongst the proteins, resulting in a lighter meringue.
The acids usually added to meringue are white wine vinegar, lemon juice, or cream of tartar. Fresh eggs are more acidic than old ones, so these help too.
Some cooks use copper bowls to make meringue, because copper ions from the bowl bind to a particular protein (conalbumin) and strengthen it.