On Google Books there's some preview pages from a book called "Microprocessor Theory and Applications with the 68000/68020 and Pentium" that seems to have a reasonably good explanation of the workings of those pins in two different chapters. Google "FC0 68881" and you'll find it. Visible in the preview is both a table of what various combinations of FCx codes translate to, and at least a brief explanation of how the FCx pins are used in concert with some of the address pins during a "CPU space cycle" to select between up to 16 different co-processors. (Motorola considered both the 68881 and the 68851 MMU as "co-processors".)

So, yeah, I guess that's why the 68030 would have them despite having the equivalent of a 68851 internally.