What about a cat who can't stand a collar? The last time I tried to put a collar on Bud was when he was a kitten, maybe three months old at the time, and he really, really, really hated it. He spent the next hour crying and trying to get it off, and at one point he let loose a shriek of frustration unlike anything I've ever heard from him. I felt bad and took it off him, and subsequent attempts didn't work either so I gave up.
It's a good idea for all cats to have some form of identification. A lot of cats that get lost are indoor cats who slipped out the door.
But I am worried about him getting lost, because he's a bold and curious cat who loves to explore.