It's a very welcome measure of goodwill from the development team, and one that will likely pay off dividends. Bethesda and id aren't the first to do this, of course -- Crackdown and certain modes of Gears of War 3 included a similar feature as well -- but it's definitely a cordial addition to Doom.
This strikes a fan-friendly balance between giving away
all of a game's add-ons and splintering the player-base between DLC haves
and have-nots. It also still allows a publisher to recoup costs after a game ships, so it will probably (hopefully?) become more common as time goes on. Where might we see it next
? This fall's Gears of War 4
on Xbox One