This is already in use, but it is just hidden from the general user.
When you click "upgrade" on an app in the AppCafe, it will only download the appropriate *.pbp (push button patch) file from the repo, rather than the whole *.pbi (push button installer). The only time that it should fetch the entire *.pbi is if the *.pbp either does not exist for the upgrade you need, or is larger that the initial *.pbi file in the first place (happens sometimes with tiny programs where everything changed).
Right now, once a new PBI goes through the approval process, *.pbp files are only created for the (I think 2) most recent PBI versions, so if you are waiting until after multiple program updates to upgrade your PBI installation, there might not be a patch file to upgrade from your current version and it will download the whole *.pbi then.
I hope this clears things up!
~ Ken Moore ~