How long do you keep old machine backups?

I'm spring cleaning the computer room. No, I've not moved anything physically. Rather, I've powered on all the old hard drives and and backing up, backing up the backups, verifying the backups are working (or have worked) etc etc.

One of the drives is a full backup of my old G5 PowerMac tower (which I need to sell. If you're interested, let me know). It was replaced by the current machine, a Mac Pro Tower in late 2008.

Why do I still have a backup of it the old G5 machine?

Do I absolutely need that backup?

I don't think I should. I "migrated" the old machine to the new (via OSX's migration tool, when you buy a new machine).

One would think that the fact it was migrated alone would alleviate the need for keeping the backup of the old machine.

Yet, here I am, looking at this drive, thinking I should format it, relabel it and use it for some other task. Have I formatted it? Nope. Will I? Probably not. Unless I need the diskspace. Which ATM I don't think I do. So the drive will continue to sit there, unused, untouched. But just incase I need a file from the old machine, which I would have deleted from the current machine, there the drive sits.

And the G5 still sits in the corner of the room, ready and packaged to be sold, and that's still not done yet either. Procrastination++.