Whether you think it's a good reason is up to you, but the reason the system clock isn't on screen persistently (it appears if you move into the corner) while a

Whether you think it's a good reason is up to you, but the reason the system clock isn't on screen persistently (it appears if you move into the corner) while a Metro app is active is because of the core design principle that says apps get 100% of the screen and system chrome gets 0%. Of course apps are free to incorporate their own clocks :)