Sorry that I cannot answer your question for the United States. But around the New Year, many European Hiltons - probably all - switched from Swisscom internet to iBAHN.

The Swisscom codes were valid for about 3 to 4 month and could be used at any Hilton. Just like you, sometimes I received a voucher, when I didn't need one. So I allways kept a spare code in my wallet, and used it when they forgot to give me one, or if the one I received was faulty or had been hacked or whatever.

I haven't yet seen a 'card' as you describe it. Until now I just got a 6-character code, often attached to - or written down on the keypack. When I tried a code (that hadn't expired yet) at a different hotel - it definately didn't work. I had to use the new code.