With so many recommendations to add zwave repeaters for a more stable network, I added 4 Aeotec repeater 7's to my hubs last year. I set everything up from scratch, pairing the repeaters first to encourage routing through them. Looking closely at my zwave details I can confirm that out of the roughly 25 zwave devices, nothing is being passed through any of the repeaters. Instead it seems like Aeotec Doorbell 6 is a popular hop, as well as the Zooz multirelay.
So more of an observation than a question, but I'd say just throwing repeaters at a system arbitrarily doesn't seem effective. And mains powered devices seem to be just as good at being repeaters.