With default resources in the drawable folder (MDPI sized) what's the point of the drawable-mdpi folder?

I've a full set of images at the various different pixel densities, and I need to put bitmap resources into the drawables folder (at MDPI) to use as default resources (to prevent crashing on pixel density devices I've not catered to, e.g. xhdpi), but that means duplicating the resources in the drawable-mdpi into the drawable folder.

So as duplication is clearly bad, I'm wondering whether I should just delete the drawable-mdpi folder, and keep my MDPI resources in the (default) drawable folder

Or am I missing something?