The Microsoft tweets seem to (probably intentionally) ignore the obvious reason why Google would want the patents — to create a mutually-assured destruction scenario that convinces Apple and Microsoft to get off its back. Sharing the patents with Apple and Microsoft would keep Google at a disadvantage just like not having them would.
Indeed, the patents used for defense have to be different from the patents you are attacked with. Sharing patents means they can't attack you with those, but if they have a lot more other patents, you still aren't protected.
