MS did. But the issue with them is their currency is it is misleading--it's hard to tell what points equals so-so money. i.e. 80 MS points = $1.00 (may have changed?). At least With Amazon, 100 points is $1.00, and the more you buy at once, you'll actually get a discount. You get a discount currently on the lowest amount.
But I agree. I don't see the point in propriety currency, especially when your credit info is still stored in order to buy it.
"Amazon adds annoying extra step to app purchasing process."
"We considered using Bitcoins, but feared the feature would actually be useful." Said an Amazon Rep. So we made up these stupid Amazon Coins instead.
Oh well, at least this seems to be optional.