The multi caliber is important if you are going to register the receiver as an SBR. It gives you the ability to shoot whatever calibers you want out of it. The ATF might have a problem shooting 9mm out of an .223 or 5.56, title 3 weapon.
I will go out on a limb here on this but................ If Congress ever requires 50-BMG rifles to be registered your Bohica, Safety Harbor, whatever brand upper will need to be on a multi caliber labeled lower or it won't be register-able due to the .223/5.56 discrepancy.
This is my best guess on the 50 BMG reasoning. I have know actual knowledge of how a Brady gun grabbers mind (if they actually have one) works.

Bottom line for me, any new AR lower that I buy must be in "multi" caliber.