As far as I'm concerned, UUIDs are the last resort of identifiers, since other than being unique they have no desirable trait whatsoever. Moreover the kernel does not even support them last time I checked, so this would force the use of an initramfs.

The mmcblk numbers (with patch) are moreover also useful to unambiguously refer to card slot itself, and the entire block device that resides there. UUIDs are not available for these purposes.

So it's the choice between a long obnoxious identifier usable for only a limited subset of use-cases, or a few-line patch to get a short and convenient identifier usable for all purposes. I don't see this as a difficult choice.