Backup systems make tradeoffs to be space efficient (block level backups) or computationally efficient (using full file backups that have mostly IO cost.)

Apple made the choice to have time machine operate with little CPU burden. While this would be a tremendously poor choice for an online storage system like SpiderOak, Dropbox, SugarSync, etc. it probably makes sense for them since it's usually working with a local external drive.

I admire much of Venti's design, but 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.