I feel that they could potentially make part of the game downloadable, such as textures and the like. Then again, I have no idea how it works. I’d say the problem with letting you download a portion by itself is it could cause corruption when the entire thing gets downloaded towards the end, which would cause you to have to download the game twice. I think it is a much better plan to have people with low download speed to play later on.
To qualify that a bit, I can’t imagine the Big Alpha lasting any less than a full week, or multiple weekends. This would mean you have at least a full 24 hours to download the game, and then 48 hours in which to play the game. Just plan your time accordingly. Do all of your weekend chores or work when the download is going on, that way you don’t have to worry about when the game is actually finished and you could instead be playing the game and having a ton of fun. Just don’t expect to be playing when you are actually downloading. Plan your play time accordingly.