I understand and have lots of NW miles flying delta. But they are merging and it does not hurt to ask. The delta letter said after the merger you will get miles from all fares and booking classes. I believe FDX pays a fare and uses several booking classes. If you remember 3rd grade english or 3rd grade math you will know some is a subset of all. I do not trust delta either. I have not merged my accounts, but it would not hurt to ask. The union asking the company to ask Delta seems an appropriate way to find out.
If we don't get delta miles I will be happy switching over to AA next year.