I have to say, I actually would find this very problematic.
1. I play D&D with my wife. We do not have two copies of every book.
2. My buddy Chris' son Joe joins us every now and then. Joe has his own PHB, but Chris does not buy two copies of every book either.
3. Growing up, brothers Ged and Ben played in my group. You can bet their parents expected them to share books.
4. My homebrew I'm working on has Mojh from Arcana Evolved. Three of the likely players already have Arcana Evolved, but two do not. Do they have to buy that book just to play in my home brew? Or do they not have all the race choices the other players do?
5. What is I try to join an online group using D&D Insider's meet-up function, and the party needs a Cleric- but for story reasons there are no Clerics, only Super-Smiters from the Complete Books of Zealots. If I don't have that book, do I not get to play?
I think all of these problems could be fixed if WotC allows you to share the book's license key with a very limited number of accounts. At least 2, but no more than 4, would be necessary, I think. Marketing research will give you more precise numbers.