I found a fairly thorough review of apt-rsync and where it stands as of 2002. It's six years old but still relevant. An interesting point it makes is that apt-get update could be handled with rsync cheaply.
For now on, I think I'll refer future advocates to that post and ask them to come up with hard numbers for costs incurred. It's important to change the status quo, not for change's sake alone, but for the improvements it brings.