I like keeping price tags on books and CDs for posterity. I have CDs dating back 10, 15 years with the label still on them, in punts. I think it's great, but it annoys a lot of people. I love playing 'guess the price' when I buy newly purchased items, but that's an aside.
On the other hand I hate the check-in tags being left on luggage. A lot of people leave the tag there until the next time they go to a check in counter. I rip it off just after I've left the baggage area.
I think the difference for me is that the luggage tag sticks out and generally doesn't fit in symmetrically as well as the book/CD price tag does.