Coming from an astronomy background, I can tell you that abso-fucking-lutely no one is going to use zettameters instead of kiloparsecs. If I tried to use that in a paper or talk people would think I was insane, and then demand I convert it to kPc.

And they would be right to do so since kPc is an immediately understandable distance (to astronomers). One parsec is roughly the distance between stars. So one kiloparsec tells me something is about 1000 star-distances across. Easy. I have no idea what a zettameter is. I like consistent rules as much as the next guy, but sometimes you have to make accommodations for things that are actually useful instead of the “strictly correct” option that is useful to no one.