As someone else has mentioned, this can be used for a multifarious amount of things other than the overpriced MW2 Map Packs. There are tons of great Xbox Live Arcade games, and even one or two Indie games that are pretty good as well (and majority are a tiny 80pts).
If you want to buy something on Xbox Live that is exactly 1200 points, then this is perfect since you will have no left over points.
If you work out how much this would cost if you could buy 1200pts via the Xbox Marketplace, since 8.50 (pounds) is 1000pts, it would end up costing 10.20 (pounds), so not much of a saving really, but this is still better if what you want to buy costs 1200pts since you will have none left over. But on the other hand, if you plan to make a purchase of more than just a 1200pts item, then I wouldn't recommend this.
All depends on what you want to buy.