you actually have 2 different product hierarchy fields in material master (PRDHA and PRODH), both make use of the same customizing but can be maintained independent of each other. One is in the sales view, the other on the general data view.
If you want enter a product hierarchy, then the product hierarchy field is probably the best choice, no matter what material type it has, I never saw any problem in using the MARA field even for material which are not meant to be sold