I do not speak in practice, but from what I studied - the purpose of using hi-ratio is because it holds a lot more sugar but still stays soft enough to pipe and smooth. And because you add so much sugar you don't taste shortening, only sweet sweet sweet. If you used just Crisco with the same amount of sugar it would be crumbly and dry to work with - and you can still taste Crisco.
But a common misunderstanding: shortening does not make icing crust. The corn starch in the powdered sugar is what makes icing crust. So it stands to reason if you are able to add more powdered sugar (which contains corn starch) into hi ratio shortening it will crust more, thus making it win=win when using it. If you make an icing with all butter and powdered sugar you will get a very thin crust, if you use Crisco you get a little more crust, if you use high ratio you get a heavier crust. I have read that you can get a better crust using 100% butter if you sift in more corn starch into your powdered sugar, but I've personally never done it.