Getting shoehorned into a tiny DLC area that you're then stuck in?
Eeeeeeewwwww! No thanks. Not being able to revisit Sierra Madre is bad enough.
Then what are you suppose to do after it? Go back to Mojave Wasteland where nothing has changed?
If this DLC takes place "after" the final quest then it can't let us roam around the mojave after we're done with it.
It would require an expansion to fit everything in the ending sliders in with the game.
Besides, you could always keep your save before Hoover Dam battle and then reload to it once you've completed the DLC.
All new weapons and items will be added to the main game's merchants though.