This will be my first time using it. In the past I have used 1/2 cup crisco 1/2 cup butter 4 cups powder sugar. 

Should I use all High Ratio in my buttercream or HR and Butter? What are the differences in your opinion? Would my buttercream crust just using HR? 

I use 2C high ratio shortening with 2# of powdered sugar.  Especially in the summer because I live in the south.  Yes, it does crust.  I also have used 1 C butter and 1 C high ratio shortening, but I usually only do that during the cooler months.

If you use a combination of butter and high ratio shortening you will notice that your frosting isn't as white as if you used only high ratio shortening.

So using half butter really serves no tasteful purpose?

Sure it does, but it all comes down to personal preference.   I think it also depends on the recipe you use.  I use a little salt with mine and usually all almond extract.  I have also used a combination of almond and vanilla extract or vanilla alone.

I have a chocolate frosting I love that uses both high ratio and butter in equal amounts, but it isn't a crusting recipe.

Same here...I use only HR shortening for my white BC...but I use equal parts of HR shortening & butter with my chocolate BC.