Difference is that the WhatsApp client re-encrypts the message with the new key from the server and re-sends it without user intervention ("non-blocking"), so even if you cared, you can't prevent it.
With the alternative, people that don't care could tick "verified" with or without verifying, but you could also click "cancel" (with or without verifying).
ALICE
When would you
like to meet?
BOB
Tomorrow, 19:00, by the
north tennis courts.
ALICE
Sounds good.
!!! BOB's key has changed !!!
BOB
Actually, could we meet
at my place? I'm going
to be super busy tomorrow.
If Alice and Bob are doing something that needs to remain secure, Alice would be a fool to trust Bob's messages after the key change without manually verifying the new key with Bob. How does withholding messages help, aside from telling the server which people have enabled the setting and which people have not? [edit: I just realized you were talking specifically about the case where manual verification is enforced for all users; disregard the last phrase.]
Admittedly one angle I can kind of agree with is that layman users may not understand the implications of a key change and the importance of out-of-band verification, and blocking messages until verification would be a way of signaling the significance of the key change. But... counterpoint to that is that users interested in security are probably already dead in the water in that regard if all they have is a layman's knowledge of how crypto works.
With the alternative, people that don't care could tick "verified" with or without verifying, but you could also click "cancel" (with or without verifying).