I don't know why, given that my CPU is a sunk cost. (Also, I suspect that Apple chose hard links instead of deltas for ease of implementation, not to save CPU cycles.)

last I checked, Venti didn't support recovering the space from deleted items, except by way of making a new copy of the file system.

I think the state of the art has moved on since Venti; Cumulus (and probably tarsnap) implements garbage collection to recover space. http://cseweb.ucsd.edu/~mvrable/cumulus/