I think Bethesda is a willing participant. X360 is their lead-platform, even when other publishers were conning journalists at the E3 by acting if they were playing on X360 but secretly using a desktop + X360 controller (Saint's Row The Third), or focussing on the PC build publicly and keeping hush-hush about console versions (Battlefield 3). It's clear other publishers feel they can't wow crowds anymore with the current console graphics and after (5) years of ignoring the PC platform now use it's graphics to captivate an audience, Bethesda stuck to using the X360 as demo-platform as it has done in the last 5 years (and didn't even release screenshots larger than the consoles 720p resolution).
At least we know what the lower bound of decent graphics will be and hope they can scale up quite a ways. I don't mind the second fiddle the PC plays, as long as they don't mess up the menu or the controls like they did with Oblivion (on the other hand there were equally bad parts in Morrowinds GUI, so being lead-platform apparently doesn't automatically mean it gets the best GUI it can).
X360 has clear advantages over PC and PS3 from the publisher's perspective.
1. PCs are pirating machines the moment you plug them to the internet, the bar for pirating on X360 is higher because one needs to modify the X360.
2. X360 is probably still the biggest player in Bethesda's homemarket. PS3 might have overtaken X360 worldwide but I believe NA is still for X360. The PC isn't even a player.
3. It's easier to demo moving an X360 debug than walking around with a desktop.
All above reasons have nothing to with DLC but simply with X360 as main/leading platform for Bethesda.
30 days between an X360 DLC release and release on the other platforms isn't that long of a timed exclusive and I don't find it's that big of a deal. One could wonder if it still adds anything to marketing tactics. Maybe for multi-platform owners to choose the X360 version or as a 'reward' for having bought an X360 all those years ago. But will it influence people to buy an X360 this late in the consoles life?