I'm not 100% sure about this, but IIRC it'll be quite a while before they plan to rework Volumes 1-3 for MMIX (the new assembly language). Instead two smaller updates have been published, "TAOCP Volume 1, Fascicle 1: MMIX" and "The MMIX Supplement" by Martin Ruckert.

Regarding if it's justified, you might want to look at Knuth's forword to The MMIX Supplement. In short, he says that it's important to have some understanding of all the layers in the computer, and thus learning assembly is time well spent even if you never use it as such.

Nope, except for the fun, the assembly part does not add much to the matter. The essence is how to design, describe and analyze algorithms, so it is largely orthogonal to the language you use.