The idea is two fold.
One, M$ spends a fraction of the money that could go towards making an actual retail game/new IP, and instead pockets the extra money, and pays Bethesda a tiny fee for a 30 day DLC timed exclusive.
Two, the hope is that it will sell more copies of Skyrim for 360, without actually adding anything to the game itself (like real content). The DLC for Skyrim was coming anyways, basically they were paid to stop the release on other platforms for 30 days. It won't actually sell more copies of Skyrim itself (across systems), this paying for timed DLC will just hopefully point users into buying the 360 version of the multiplat over PC or PS3. The downside is...retail we get one of the following from that 1st party : GeoWHaloFable. Good 1st party games, but it's slim pickings until the end of every year, where we get to choose one-two of those offerings. How about that timed DLC in lieu of new retail games? Eh? :P
There is always fun multiplats, but my point is that this TIMED DLC has a cost to the gamer, whether you buy DLC or not.