Or... Worst title ever.
Whenever I have to provide my credit card information, I like seeing the dropdown boxes for the expiration date. While I doubt there's a keylogger on my computer, it gives me a little extra sense of security.
Spaf said that making an online transaction is like giving a bum a quarter and transporting it in an armored truck; the encryption will keep your information safe--the weak points are you and the merchant.
If your computer happened to have a keylogger installed, the thief would be able to easily figure out your credit card number, but tracking mouse movements and clicks, and trying to piece together what month you clicked is much harder. If you change something as simple as the font size, use a different browser, or briefly do something in another window, determining the expiration date is much harder.
Sure, within 50 attempts, the thief could probably guess it, but credit card companies are always looking for fraud, and trying different expiration dates that aren't close to the real one sends up a red flag, hopefully saving you the headache of cleaning up your credit.
Or you can not run that .exe you were emailed: your choice.