Mostly, and this is just my own limited, personal experience, MTB stuff is designed
for much wider ranges than your average road (non touring).

So I guess I'd want to know why, exactly, you'd want to do this (other than cheap
on e-bay ......and just so you know, I loathe e-bay)?

Again, from personal, limited experience only, the smaller range stuff will
shift and work better on a road built up for fast riding.YMMV