In some cases there is not even a signature to check! The classic case is using AMEX to buy fuel from the bowser. Swipe, fill and go. Since AMEX have always been brilliant when I've had disputed charges, it never really worried me.
Having had my CC stopped a couple of times due to fraud (not mine obviously
, I spent a bit of time investigating how the banks check for it. I had a nice long chat with a Westpac fraud investigator while waiting for some forms and he explained a few niceties of the system. All of the majors have fairly sophisticated software that tracks usage patterns and compares them to what is 'predicted' for that card. Westpac even adopted a policy of 'cancel first then query later' for certain transactions since they had such a good hit rate.
I asked him about the signature requirement and he said that the push to check signature you see in big supermarkets / dept. stores is generally driven by the stores themselves. Since the merchant wears the cost of the false transaction, it's in their interest.