Very interesting dilemma…. I predict that in my community in the near future, the topic/PM split will be 50/50. I can see why for some communities it would be highly uneconomical to include PMs in anything AI related and why in others it would be invaluable. Perhaps it would be ideal to have a method by which discourse admins could decide from the very beginning whether or not to include PMs? This way, there can be an executive decision made based on the community type because sometimes AI has no role to play when it comes to basic question/answer PMs, and other times it may be critical if the community shares searchable/valuable information via PM.
Nonetheless, search is key here: does your community type benefit from generating embedding for PMs or not? In the end, it requires an ultimate decision I think