So, it still has advantages to have multiple accounts, they are just not as prominent as in other schemes.
Well, that entirely depends on the chosen incentive mechanism. If you opt for the "mine to be released from some duty" model, multiple accounts even become counter-productive. For example, if every member has to perform computional work to stay alive (e.g. by regularly submitting PoW-backed transactions that get included in the blockchain), and by mining a block you get released from that duty, then owning several accounts would result in a multiplication of the required work. In such a setting, it doesn't make any sense to have multiple account since the rewards are calculated based on your total balance, no matter how many accounts you use to store it.
It basically works like Lisks/Arks DPoS system, with specific accounts, which are allowed to contribute to the blockchain (and mint coins in the process). Different to DPoS, who is eligible for minting is determined by a token which differs from the regular coin: If an account holds such a token, it is allowed to mint.
Thanks for the hint. I will take a look at Lisks/Arks DPoS system.
The specifics about how a token is accquired and what "minting" means exactly seems to be up for discussion, although the author gives some ideas, which are quite interesting. Overall, not a bad concept.
Maybe, you can come up with some more ideas/alternative incentive models? Let's have a discussion.

This looks like an idea, which looks interesting especially for semi-private blockchains, blockchain-as-a-service solutions and so on.
I think, Proof of Membership could be used with both permissioned (closed) and permissionless models as it only limits the number (creation rate) of accounts, whereas everyone (who can afford to make the highest bid) could become a member.