Cost of the bandwidth is one issue. The amount of time it takes to download is another. I have a 4mbps connection but power cuts and fluctuations makes it difficult to do a 4gb download from Apple. Every upgrade of Xcode is a horror. The download stalls in the middle and then fails to re-continue because the session timed out. I have to end up doing some gymnastics by exporting cookies from the browser to a file and then coaxing wget to use that.

I hope they've got a decent pause/resume system on the Mac App Store downloads.