I just remembered, I also bought the StarCraft 2 expansion last year and it seems that all that was on the disc was a code to unlock the game for download. If I recall, I don't remember data files being on the disc. When I entered the code the unlock the game, it proceeded to download game files. Pretty much paid for a disc with a code in it, if I remember correctly.
Someone will upload games on internet. As a social being, human needs lot of money to buy this and that. So I think it will be wise to keep your money for something more important than bought a physical copy of game that you can get it for free in internet. Download it and don't forget to say thanks to the uploader. (Personal opinion, please don't mind)