Back when luggage tags had some type of identifying information (name, Delta Frequent Flyer/SkyMiles number, etc), these tags served a useful purpose. Now that they no longer have any PII on them, they're fairly useless. I use a label maker and clear tape to make a label with my SkyMiles number and put that on them so that they're useful. The metal ones are somewhat more durable (unless taken) so that's a nice thing that Delta has done for MMs.