Gay/r and WTF/r both have trouble with Crucible, if it resolves,
Every deck in the format but pure combo has trouble with a resolved crucible.
Many of you seem to be advocating MD oxidize primarily as a way of dealing with Crucible of Worlds.
Cruicible is just one of the mana things it deals with, and is a good example because it is a troublesome card that is quite scary without some sort of maindeck removal for it. It kills random artifact fatties, and all of the stuff without activated abilities that rod cannot shut down. Cruicle will not answer a gigANTIC beatstick ie 7/10, pentavus, etc.
As I said I am not opposed to 1-2 COWs in fish, however I think that removing creatures for them is a bad idea. I would probably run 1 in a daze/stifle/mis-d slot.