There are needed in my opinion.
Not only you get immediate consequences of your actions that would be hard to depict in a game engine (Vault City annexion by NCR in Fo2, various endings of New Vegas...) but also some events that would take a lot of time to actually happen, (the creation of the NCR and the end of Fo1). Some of them are stories on their own and would probably need another game to be properly fullfilled (Barnaky's Brain Ending in FoT). Also, there are some consequences that you wouldn't want to see ingame (Myron killed in Fo2, or the Kings slaughtered in New Vegas).
Also, it is entirelly subjective, but i am quite addicted to extended endinng. Nothing is more frustrating in a game, to just have the credits rolling after defeating the big bad boss. The same could be said about very long movies or TV shows that don't have a proper epilogue. So much time fighting to solve a problem, and no time to see the results ? Are you telling that they did all that for nothing ?