That's just STUPID. At the minimum, you should be symlinking from the new backups to the previous backups for things that haven't changed, and then when you expire a backup you roll any files forward until you hit a non-symlink. Jesus, this is RIDICULOUS.
Ok, I'm only using about 12 gigs right now, but what happens when I've filled my 750GB drive? You won't be able to keep two whole backups on my 1TB backup disk. Do you get smarter as you run out of disk space?