this is just a way to encourage network operators (who want to play nice) to run more than a middle node without a lot of overhead. Or do I misunderstand?
You're fine. I was only speaking of the internet path between the exit and regular internet services. Such as what happens if a Tier-1/2 props up some fast nodes and might be able to vacuum up Tor traffic destined to itself. How that would affect the current presumption of reasonably randomized exit sources of traffic to that AS. Mostly from an abuse jurisdiction standpoint as it's harder to complain about (or apply policy to) roving exits halfway around the globe than it is one on your front door. Yet since anyone can prop up such a server today, it's moot I guess. That leaves just the Tor traffic skewing to consider. Also similarly mooted.
(My understanding of an adversary being able to effectively force traffic to a destination through their exit... is weak. But if so, that's the skewing part of this.)
Would this also mean that ISP's (or users) who deploy such a node would be more likely to block all other nodes such that all they need to manage/filter/etc, good or bad, is their own 'peering' node? Whether such nodes reside in/out of their AS, or use 0.0.0.0/0 exit or AS only exit.
Is all.