My best guess would be that because when wow was originally designed the technology didn't exist to keep the game up forever like it does in other games today. You've got to remember that wow is almost 10 yeas old now, it's made a lot of advances and changes but a lot of the base code is still there so it needs to have things rebooted and restarted to make sure everything is running properly. As far as the actual maintenance, they're probably putting in things from the PTR slowly piece by piece instead of all at once. They also use maintenance times to straighten out hot fixes they make in between Tuesdays, like last week they made the hotfixes to hunters, they're probably take this time to change tooltips and everything to reflect the change.