All I know is that when I see an older road bike on CL, the seller sometimes seeks to extract a premium price because it is "vintage" or "rare." Sometimes they are; usually they're not. I've never seen a seller on CL try to get a premium price for an old mtb. The market, at least where I live, treats the two bikes differently. Seattle might well be different, however, when it comes to vintage mtbs than where I live in the middle of the US of A. Certainly the coffee is better and the food in Seattle! I think it is a very cool bike.
I've made all sorts of bikes into commuters or beater bikes over the years. My rule of thumb is that it has to be cheap (since it might get stolen) and it has to be good (why ride a cr##py bike?). I've used road bikes, commuter bikes, three speeds, you name it. Vintage MTBs are my favorite. They're stout bikes with forgiving gearing and fat tires. Plus the older bikes tended to have more relaxed geometry than the modern ones which also helps out.