My friend has been playing Diablo 3 by simply copying the Diablo III folder from my hard drive. Seems pretty portable.

By files, do you mean personal files, or the game files? 
The game itself can be downloaded offline, and since you need online to play, I don't understand.

Hello you fine folk,

Now that the summer looms, I think I'll buy D3. However, I've a data cap on my 'tubes for the summer (feh) and so I'd like to wonder aloud at the potential feasibility of purchasing a license, obtaining some or all of the files from a buddy's computer via USB, and hopefully only downloading the latest patches/verifying my new data. Does anyone have any thoughts on how well or poorly this might work?

Question about D3's portability