Well, leaving old tags on I don't think hurts anything.

What I'm getting at is that if I use a new tag, and the airline LOSES my bag, I can 'prove' that I gave them a bag because I have the stub of a recently issues claim ticket.

But if I reuse an old tag, sure, if the airline MISPLACES the bag, they can use my old stub to match it up (but they could also just use my business card in the bag for that), but I can't really 'prove' that I gave them a bag - so if they LOSE the bag, I can show them my old stub, and they can just say "that's nice, but showing us a stub from last year doesn't mean you gave us a bag today".