Why should families be allowed special dispensation? If I live in a house with 3 friends should we only be allowed to buy one copy and share it amongst us? When I was young....I got my own games.
I don't know your age, but when I was young games came on cassettes at £0.99 or £1.99. Not upwards of £40 as it is now.
If MS was able to link accounts based on billing address then fine, you and your friends should be able to buy just one copy.
Either way, both scenario's are slightly different.