From personal experience:
- if you have a heavier metal card and you drop it, it usually makes a louder sound so you are more likely to realize it dropped and pick it up before you lose it :)
- a metal card tends to scratch and rub-off less than plastic cards I've had in the past, so they are less likely to become illegible and beaten-looking before their regular expiry date
- cashiers occasionally comment on how heavy the card is, so it gives you something to chit-chat about for five seconds other than the weather
- it feels a little nicer to find, remove, and put back when I need to use a card, and since there are so many cards with the exact same cash back benefits (even considering ones with no user/monthly/annual fees), its a reason to prefer one card over another other since there is no actually good reason left
Grand total value: I think if I had the option to pay for a metal card over a regular one, I'd consider maybe paying up to $20 (per card, not annually). If the price was more than $50 and I'd feel silly for the meager benefits of the material.
I got a credit card offer a little while ago trying to convince me to get a new card because it was a "luxurious, black metal" card, and of course the annual fee was around $500. I had a good giggle over that for a while, and then I realized some people might be convinced by that and it made me a bit sad. So much effort put into convincing people to give up lots of money in exchange for so little benefit.
Its just a little square that lets you owe other people money.