For Bethesda themselves there is. Microsoft and Sony charge developers for patches, so releasing two patches shortly after each other costs them more money.
It wouldn't surprise me if the 360 hadn't gotten the 1.6 update either, if it hadn't been required for Dawnguard to work.
That could be avoided by not releasing a game with so many bugs, thereby not needing to have so many patches.
Also, the cost of the two patches is a sound investment to encourage faith in the company. Withholding patches can generate a lot of ill-will, and that can manifest itself as loss of sales on the disenfranchised platform for future products.