This is an attempt at a unification of:
I can't think, nor have I heard, of any showstopper reason for why BitTorrent shouldn't be used as the primary download method of Ubuntu respository packages. Although the specifics of the implementation of this idea will be different for ISOs and repositories, I feel they should be unified in the brainstorm because the goal is to allow the rapid, efficient, reliable, and available download of Ubuntu software.
1) Speed. All Ubuntu downloads (ISO downloads, dist upgrades, regular system updates, and new application installs) will as a whole be faster. Generally torrent download speeds benefit from higher numbers of downloaders that seed, which Ubuntu users have demonstrated they are prone to do. BitTorrent is better able to absorb (and eventually use as an asset) large numbers of users attempting to download data at the same time, such as with the recent mad rush of Hardy downloaders/upgraders.
2) Efficiency. The BitTorrent protocol has proven to be one of the most efficient methods of distributing data amongst a large number of clients. It will harness the collective upstream of tens of thousands of Ubuntu users, from DSL and cable connections to the fastest of corporate connections.
3) Reliability. Checksums guarantee the integrity of BitTorrent downloads, so data corruption is much less likely to occur. Only the pieces that fail checksum are redownloaded, contributing to points 1 and 2.