Actually, there's at least one very good reason why MS's implementation is a good idea. You just have to look at it from a point of view other than your own.
Under 18's can't have a credit card. Sure, their parents could buy them some MS points using their card, but there's another option available:
The under-18 with a desparate need to play the full version of Uno on Xbox live, or buy the latest depressing Emo song (whatever the kids listen to these days, I've no idea) can also go into a brick-and-mortar store and buy an MS points voucher with cash, which they then associate with their account, no credit card needed at all.
I imagine there would be a pretty huge percentage of under-18s who would love to use ITMS if it wasn't for the credit card thing. MS is going to attract them in droves. It's not that much different to buying time in bulk for your Pay-as-you-go mobile phone.
Zune Marketplace's Absurd Pricing Scheme