I don't want a half-dozen, I want six.
From a business perspective (accounting really) there's a pretty big difference.
Something isn't spillage if it's claimed. Usually, when purchases like this are activated through an account, left over change isn't considered spillage unless it can expire so you lose it. With gift cards, people throw them away when they have an amount they consider unusable on it, averaging something like three dollars. That is gone forever, and just becomes profit from no service being rendered.
Point block purchases, like this, are a bit different because you always retain the points in your account and it's assumed there is going to be an inflow and outflow of money in that account. It isn't spillage unless the account is rendered inactive. The number may not line up evenly, but the points are flowing through so it can't really be said that there are stagnant dollars in there. Generally, this is like front load invoicing, and done for cash flow reasons (the chance of free profit is awesome as well). The thing is that with point block systems, if they were really going for the spillage, they'd just mess with the fake currency exchange rate in order to automatically include the spillage (the value is already obscured, 400 point purchases would now be 500).
Basically, spillage is free profit, point systems like this are revenue control mechanisms (generally). Don't get me wrong, it doesn't make it any more convenient - it's just a different motive and viewed as being somewhat more ethical.
A comparison would be the system that Skype used until very recently. It was point-block, but based on currency. You had to buy a predefined amount ($10 US), but you could reload another block of that amount onto your account at any time. In that respect it was front loading. It'd be easier for them to bill your card or account, but this way they had revenue to show up front and cash flow to work with before the product was used. What Skype got in trouble for, though, was they had it set up so your points would expire after 6 months of non-use. That was there for the spillage and is pretty much illegal in most places.
Reading on this class action suit, it doesn't seem that they're going after Microsoft for the point system itself, but rather because of issues associated with it - like getting charged for downloads you've cancelled.
We'll see how it goes, but they really should switch the system over because it doesn't do them any good. People tend to stay away from point block systems if they can because they're a headache to actually figure out a lot of times.
