My first concern is the loss of information when the route to M isn't announced. This causes unfairness when traffic ends up taking the 'long' route.
My peer fears that and would like me to fix it. I don't understand how I can do that in a simple maintainable fashion.
More than likely your peer is doing the same thing unto you.
Quite possibly, but they won't 'fess up to it. And I don't want to whine at them unless I know how to constructively address the opportunity (the peer is a Californian:-).
The second effect of M's route not being announced happens when traffic is blocked because no 'longer' path shows up anywhere else due to different route weightings and policy filters across various combinations of ASs. I consider this possibility the more serious problem. As the peering mesh becomes sparser, expect more missing in action paths, even if the physical connections exist the 'best' path may not be announced.
If my peer does not agree that my policy is reasonable and a consequence of current tools, their reaction may be to reject inconsistent announcements thereby increasing the likelihood that no path is propagated. randy