I usually forget to sign the back of my credit cards. Or, with one of my cards — the one I use most frequently — the signature rubs off quickly. Every now and then the card will be rejected because it doesn’t have my signature on it. Or they will require ID.
I then offer to sign the card, but they never accept this possibility. Hrrmph.
Could not a thief have signed a previously unsigned card before using it? In fact I would expect precisely that behavior from a thief. Wouldn’t a thief take more care to sign than would a lazy, careless card holder? Upon seeing the unsigned credit card, their estimate of my honesty should go up not down (well, that’s not quite a stable equilibrium…).
I have wondered why it ever makes sense for cards to be signed. If you sign a card and it is stolen, can they not forge your signature more rather than less easily? (Imagine signing into one of those signature-reading machines.) And if merchants were more rational, maybe the signature would carry no positive value in the first place.