Time Machine Disk spinning all the time?

Well, believe it or not disks exhibit less wear when they're spinning as opposed to when they are constantly put to sleep and woken up.. Kind of like a light bulb, which will more likely burn out by being turned on and off all the time, rather than staying constantly lit. Simple physics.

Since the time capsule is technically a server, even claiming to use "server grade hard drives," Apple is probably just abiding by this principle and keeping the drive always spinning.