Why do you think people lease and mortgage purchases that cost them several times more than the actual value of the purchase? A year's subscription to WoW may cost you $180, but when you're paying it in $15 increments, it becomes easier to make that payment. $180 up front is seen as a more significant cost than $180 spread out over 12 months. Furthermore, saving up for $180 does have an effect on people...since it becomes easier to impulse buy something that comes up. Now, I could save up for 12 months to buy the video card to play the game I want to play (net 0 months of play time), or I could just play the game (net 12 months of play time). Which one do you think someone that doesn't have much for money laying around would prefer?
If I plop down $180 on a video card, but then hit a financial crisis where I'm very tight for cash...I am pooched....the money is gone, and never coming back. If I am currently in month 3 of my $15 subscription, I stop my subscription, and still have money left over.