Aha!

I have figured out why you need the Mac OS X startup disk to boot off of a Time Machine backup! You need the disk because, depending on your settings, Time Machine may not back up your system files, and without your system files you cannot boot off of the disk. To simplify things, Apple made it so there are no Time Machine boot ups. Also, to add to complexity, multiple computers can be backed up on a single disk; it creates a folder for each computer, so booting off of a Backup disk could get confusing.

Also, with regards to not being able to back up onto the same disk: you can. Create a new partition using the new dynamic partition resizing and point Time Machine at that partition. Voila.