Obsidian loses points though for making the end of the game contained in a DLC. I don't know if they had the original ending and decided they could do better (which would be more legitimate) or if they decided they'd be losing money to put all that content in one game (less respectable) or if they decided they could squeeze more out of us by breaking it up (woudn't put it past them).
However it happened, it was a good game and I didn't think twice about buying New Vegas.
Are you referring to the Fallout 3 Broken Steel DLC, which raised the level cap to 30 and allowed the game to continue past the original cutscene-to-menu ending once the main plotline had been completed? If so, I think you meant "Bethesda", as Obsidian are only responsible for developing New Vegas.
As far as I can recall, the decision to expand the "endgame" in Broken Steel came about as a result of request from fans who wanted to continue playing past the conclusion of the story. I seem to remember some BethSoft employee being quoted as saying they never anticipated that people would enjoy their game that much...
I came, I saw, I deleted all your files.