In general all fields in the material master tables have their desired usage.
It might be that there is the one or other field which is not used in your current design, but who knows today if you need to use it in future?
I made some experience, our company is an assembly of several earlier independent companies. Each one had their own IT department and SAP system. Our company is still growing with acquisitions of other companies which have as well their own SAP system.
We are in the process of merging all together into one big ERP system, my main job for the last 10 years. There is no project where we are not faced with content in fields which does not fit to meaning of content in the same field of the central system. It always comes to a decision if this data is still relevant for their future business, and then we have to find a place for that.
Adding an append field to MARA table would cause the recompilation of programs, which actually means downtime for the users. (they still can try working, but may lose their data entries and cause a lot dumps) On a machine that hosts companies from all over the world is never a timespot without affecting any business, not even on weekends.
So we early needed a strategy. And this is: no more Z-Fields in table appends, no misuse of SAP standard fields. Any extra required field is done with classification