rabies vaccine tag?

That's the reason they issue the tag. A strange, free-roaming cat could bite or scratch someone, and that someone might especially appreciate knowing that the cat has been vaccinated. (That's a hypothetical, of course - I have no reason to believe your cat would do any such thing! ).

My vet gives me the tags as well, and I don't use them either; I just keep them on file with the written proof of vaccination. (In any case, all of my cats are kept indoors.) I *think* some municipalities require that dogs wear them when outside; I'm not sure about cats.

Some places do require them on the animal... but Realize that Piece of Paper will still be needed in case anything happens.. See many not so honest folks will switch out tags to an animal not vacinated ... So in case of bite ( most places it has to be a HUGE bloody scratch) you can have the tag but that just gives the AC a way to get a hold of you or the vet to see the paper///

My boy wears his rabies tag He has a few tags on his collar and doesn't mind. (( he must wear his license at all times, so it is no big deal to add the rabies tag as well as micro chip tag - I have all 3 on one ring))

Most of the dogs I know wear their rabies tags. My dogs do, but I use tag silencers so they don't jingle-- we're in Chicago too, my dogs have ID tag, city license, rabies tag and microchip tag to wear! Actually the last time I got a city license for my dogs it came as a sticker that goes on the rabies tag instead of a separate tag.
Actually Cook County law says that animals must wear their rabies tag on their collar or harness whenever they're off the owner's property. However most people I know don't follow this law for their cats. My (indoor only) cats do wear collars with ID tags, but don't wear their rabies tags. The ID tags I use are CollarTags which attach to the collar like a nameplate so there is no jingling or anything hanging down...
My vet usually tapes the rabies tag to the rabies vaccination certificate, so I often just leave it like that and put it in my filing cabinet.

Actually a rabies tag can also be used to locate you if your cat/dog is missing... The Cook County tags have a phone number on them of Animal Control, and they can give the info of the vet who issued the tag, and the vets have records of the rabies vacc numbers. I've done this before when I found stray dogs who only had a rabies tag and it allowed me to reunite them with their owners (except in one case where the vet had only seen the dog once almost a year before, and the contact info they had for the owners was out of date...)