In the instance of an unsigned card, I have had experience with the card being refused for being unsigned. I have never had this happen in the US, though, as has been pointed out, it is within the rights of the merchant to do so, but I did have it happen in England while I was on vacation there.

There is nothing more panic inspiring as being in a foriegn country and being unable to access money because you are being told that your card is invalid. I had to leave that bank, sign the card, and find a different bank to obtain the funds I needed.

All my cards are now signed and have the request to request picture ID under the signature. I personally say thank you to every clerk that checks and asks.