I don't get why people say that we would have to "re-download" Dawnguard. I'm pretty sure it would work just like any other DLC that gets released. When you get map packs for multiplayer games, they don't make you re-download the map pack, they patch it.
map packs are something completely different then expansions. You can't load a map if you haven't downloaded it do giving every one the patch means nothing but real DLC expansions are to woven into the base game for that to work. If your going to make a comparison compare similar style DLC and nothing from the PC. versions