I'm especially interested in hearing what Quicken Mac lacks in its existing brokerage accounts.
From what I have seen the main reason that Quicken Windows got this feature is because its brokerage accounts were first a "list" that changed into a very restrictive brokerage account "register" (To this day the documentation still calls it a list).
Anyone that thinks because they see some kind of checking account features at their broker that this is a reflection of this is fooling themselves. When available (not for retirement accounts for instance) it doesn't matter what is at the financial institution. The truth behind the scenes is that the transactions are all downloaded into the investment account and cash transactions are then pushed/pulled from this virtual checking account.
A side note of asking this question is because I think there are "side effects" of having such a virtual account that haven't been specified in the request for this feature, like having the account appear separately in different parts of Quicken that the Quicken Mac developers might not be aware of and as such when they develop this feature this might not be implemented.