True... but you can only Mime your own limit, wheras anybody can mime (and equip) KotR. Also with KotR you can afford to break the chain by healing if you have Master Summon or more than one star, but with Omnislash you're SOL for a while if you break the chain.

Plus, much as I hate to admit it, KotR is stronger than Omnislash against multiple targets since it always hits all enemies. (But Omnislash remains champion against a single opponent.)

Edit: If it makes you feel any better (don't know why it would tho), I DID use KotR + Mime to kill Ruby, because Omnislash only does about 15000 a shot against him and there's no other f**kin way to do enough damage fast enough.