MC's (like other CC 3-4 digit codes) are there to ensure that the purchaser is in fact in possession of the credit card being used.

In the past, Priceline, as well as other travel sites, were victims of credit card fraud that cost them millions of dollars.

Expedia lost about $60 million at one point last year when people used stolen CC's to purchase plane tickets on their site.

Priceline was vulnerable to CC fraud because you could buy hotel rooms and plane tickets in other people's names. Therefore, the crooks would use a person's stolen CC info and name to buy tickets/rooms in their name(no doubt using a fake ID).