I can see the need to protect yourself. There are careful people. I am just curious how anyone would be able to hack your account without it being your fault. They have to guess a name they have no idea what it could be, followed by an even harder guess at something that should be personal and only you should know. I kinda think if you are dumb enough to let someone be able to get into your account you deserve it.
On the other hand, I know there are people who are not as fortunate and play on public computers and there I see a chance for an honest mistake, like leaving it up to go to the bathroom.
I could see this pin system implemented I am just very torn between it helping those who are carefull and helping those who could care less.