Is there a specific reason why the macro NMBCLUSTERS is used
directly (e.g., to define VM_MBUF_SIZE, and in machdep.c),
rather than using the variable nmbclusters?

It's not like the extra three or four memory references would have
a noticeable cost; and it would be easier to change on-the-fly,
so to speak, if all usages referred to nmbclusters, instead
of NMBCLUSTERS.

Or is tech-kern the right place to ask?