This is for someone else, just bought a 1TB iMac. I spec'd a 3 TB TM drive, only because it was $20 more than the 2 TB. It would be very nice to put the contents of the computer being replaced on a partition of the TM drive for easy access. The old computer is such a mess, they will not be migrating but starting fresh. The alternative was pulling the old drive and putting it in an enclosure.
You're about twice as likely to need your time machine to do a full disk restore following a hard drive failure or partition table crash than from damage to your data partition or crash of your OS.
So using another partition on the same physical drive is probably a bad idea. The backup is lost as well if your hard drive dies. Unless the only thing you intend to do with TM is fish out files you lost or accidentally deleted, get your TM backup onto another physical drive.
