I did a little bit. While we could try and use libgenunix.so as a source for things, that kind of implies doing it for eveyr possible depenency tree. Arguably maintaining the mapfile as a source of what's public may be worse. But there didn't seem to be an easy way to do this initially, at least. I also like that the default install target verifies that this is the case and ld complaining while building the module is pretty clear.

Part of the reason I did it this way was to allow us to over time, be more concrete about what we considered available from different portions of the system and make it easier to see what folks used. That way one can distinguish between say, igb using MAC as a device driver and what dls uses that's private from MAC. That may not be worthwhile.
The build system is a large part of the design here. This basically allows us to do builds in parallel, without adding dependencies between kernel modules that match the actual dependencies (which given modstubs, isn't pretty).