Amex has updated its Membership Rewards terms so new cards get separate rewards accounts that you can link manually. It's minor if you pay attention, but the motive is unclear.
Sneaky, sneaky. They want you to lose what you accumulate on one product, when you forget to transfer before closing a card. (And, if not that, rest assured, the Amex RAT always gets fed... 'HISS!! ...nom-nom-nom-nom.')
There could be some value to some people in keeping rewards earned on business cards separate from personal
Aside from the business owners that may definitely need to keep personal and business MR cards separate (i.e. tax implications), I guess you can convince me that Amex will eventually require someone to have a Platinum/Centurion (or maybe a Gold) to transfer to Delta 1:1 or maybe also Hilton 1:2 and Marriott 1:1 as now. And then you wouldn't be able to do that with legacy Green or BBP or legacy ED/EDP.
Otherwise, I really don't see where Amex can really incentive holding a premium card to unlock better ratios for transfer partners- most people outside these websites/forums probably don't care that much about redeeming through Aeroplan or Flying Blue or Avios (and IMHO they likely aren't). But they do recognize Delta and Hilton and Marriott (not the most valuable by any means, but "familiar" brands that are pretty easy to earn/redeem with).
Hence why Citi makes you hold at least a $95 premium card to get 1:1 AA transfers and Chase likewise with UA and WN- they are the most popular/obvious reason that most cardholders interested in travel rewards would ever apply for Citi or Chase credit cards.
How are there tax implications of pooling personal and business miles/points for a single member LLC (S-Corp or Sole Proprietor/disregarded entity)? I understand that cards are ideally kept completely separate for spending and payments of balances, but credit card rewards are not taxed and valued at zero.