Heehee, guess we will have to be careful about using the terms stabilize, set, crust.
When you make old fashioned Royal icing using egg whites, now mainly only used where there is no risk of it being eaten, strictly for decorative purposes, it will help stabilize the egg whites especially when the humidity is really high. Don't know if folks have had this experience, but I have. Sometimes the egg whites just want to separate shortly after they have been whipped up. Cream of tartar helps to keep them combined and stop this separating watering out action. Therefore your Royal Icing will crust or set or be more stable, whatever term folks want to use.
Until recently, I have only seen it called for in Meringe type toppings and icings and in whipping cream and such, never mind all of the other reasons it is used in baking, we are talking icing and toppings, haha! Anyway, recently it is appearing in regular old so called "American Buttercreams", the kind where no egg whites are used. So when I referred to crusting, perhaps I should have said stabilizing or setting and it does indeed have this action to some degree. It is again being recommended in areas of high humidity just as adding flour has been suggested in these climates.
I am usually careful to use these individual terms but I note that most folks tend to use them interchangably. For example people refer to icings using butter and shortening and cream as crusting whereas I find that in practice these icings set but do not "crust" at least not by the definition commonly employed, as is firm up or crust on the outside while remaining soft and creamy underneath.
I thought that this was the kind of recipe the original post was all about as meringue buttercream was not the term used. I too have seen it recommended to be used in regular buttercream recipes on several sites in several posts.
Since the standard amount added is 1/4 teaspoon, I would experiment with this amount to see if you find it makes a difference.
I have also found several posts where folks are recommending the addition of cornstarch to buttercream icings again as a setting or stabilizing effect in humid conditions.
If the original post was referring to these meringue buttercreams, I apologize for my misinterpretation.
Hugs Squirrelly Cakes