Interesting, America being different again
I found http://krebsonsecurity.com/201... which has some background. I'm not so sure about the security risk, there has been a recent slight increase in lost+stolen fraud in the UK (PDF graph, starting at £120M pre-introduction it reduced to £50M, but has since increased to £60M. (In step with other types too, so maybe it's just more crime in general.) That contradicts the person quoted though.
The other suggestion -- that people will pick the 'easiest' card in a competitive market -- sounds much more likely, especially as it's the reward cards that do use a PIN.
Not having a retailer take a card to check a signature helps -- they can't copy down the details to use online. In restaurants they must bring the machine to you, so you can type a PIN, and the card never leaves your sight (or often possession).
The only possible interpretation of any research whatever in the `social sciences' is: some do, some don't. -- Ernest Rutherford