Many of the recipes for roast marrow bones suggest a short (20min), high temp roast. Are there any reasons to cook them at such a high temperature instead of, say one hour at 350°F?
Bone marrow is made up of a large portion of fat. If you roast it low and slow, you render out a good portion of that fat and are left with a liquidy mess. Also the high heat will start to carmelize the bone marrow, providing much flavor.
I don't see why you can't cook your bones as low as 350F. I've seen roasting temperatures all over the place in recipes for making stock, between 350 and 450. However, roasting the bones for longer doesn't really buy you anything -- it's not like you're roasting a meat, where you want connective tissue to break down over a long period of time. So the question I would pose is: why do you want to do it this way?