This is the biggest downside in my opinion. Theoretically any merchant should deny any card without a signature, End of Story. In the merchant agreement they sign with Visa and MasterCard, they agree that any fraudulent charges where they did not check the signature are their responsibility.
How often are the cashiers checking the back of the card? Many of the card readers I see are on the customers' side of the counter, and I swipe it myself, often because I'll enter a pin. Also there are self-checkout kiosks with little or no supervision. I don't think that the lack of a signature line is the _biggest_ downside.
