Well, since they only have to drive 1080p at 60Hz with most of the shaders turned off, they don't need near as much processing power as hardcore gamers running dual 1920's at 120Hz with all the options turned up to 11 with their music player, teamspeak and a web browser with item maps and strategy guides all going in the background.
Consoles never have been and probably never will be as powerful as a serious gaming rig.
Heck, Infamous (1 or 2, don't remember which) doesn't even show a start menu, it just loads directly into game and starts you in a safe location in case you're getting something from the kitchen while it loads.
Now Assassin's Creed on the other hand... Sweet Jesus it took me 10 minutes just to figure out how to quit the PC version the first time.
If it's like any of the other Microsoft installation limits I've run into, you just call the support number, they ask you why you're installing multiple times, and you tell them you fixed the computer because you the repair shop replaced your motherboard and hard drive. They are pretty reasonable in practice.
The fact that people consider this reasonable still boggles my mind. If ford required you to phone them and re-activate your stereo every time you replaced your spark plugs, there would be a fucking media storm so big their stocks would drop faster than an f-150 driven off a cliff!
You're assuming that you will only be able to purchase physical, ship-able things. I'm sure you'll also be able to buy virtual gift cards, WoW subscriptions, cell phone plans, game purchases, iTunes, stocks, etc. With the current (you must give the companies your CC number), only the companies you have done business with can take your money. With the twitter thing, ANY business that does transactions over twitter could potentially take your money (or people use your money to get stuff from them).
Never say you know a man until you have divided an inheritance with him.