The Passport option costs a measly five dollars, and nets you three full days of gameplay per 30-day period. As the announcement makes clear, the option is ideally suited to those with an occasional interest in the game. Maybe you want to make sure you get to all the seasonal events, play with your friends one weekend a month, or bring up an alt account to help you out on a raid. With the new Passport that's all doable, and affordable.
As a bonus, you can make use of the Passport to log in for special 'claim' items without having an active account. The only caveat is that the Passport is only available to existing inactive or closed accounts.
Check out the press release on the official site for more details.