Well, if you download the alpha for free, you don't have a premium user, so you can't play on most multiplayer servers, but you get the full singleplayer version for free if you update it by using "force update". You will have the latest version, but it will still say you have version 1.3. So otherwise yes, if you want to play multiplayer and you want it to say you have the latest version. So if you buy it, you have the real game, not the cheap free download you can get from the web. Then, since you have that version, you have the full game, you've bought it, and you have nothing to worry about. So, the answer is, no you don't.
