I have used this to my advantage so many times because people expect to see you with one type of weapon the bam! All of a sudden you have a different weapon!
It throws them off and you get a kill.
in my head i imagined some fat, hairless dude sitting at a computer playing tf2, seeing a pyro, expecting a regular-ish flamethrower, seeing the cow-patterned one and having a heart attack
it was pretty funny