I was actually really annoyed by that quote due to my play style and what I find enjoyable in games. I am completionist, almost to an obsessive degree, and will grind through every sidequest to get all the content, which made playing ME1 and AC games just awesome at times. However, at the same time I really enjoy stories and find it extremely rewarding to watch those final credits roll after hopefully a great and emotional ending, being able to just walk away from the game with that high point.
Now the reason that quote annoyed me is not that the world remains open and playable after the ending, a lot of games do that, Tomb Raider, SR and AC games come to mind instantly, but rather that their design philosophy seems to be that some parts of the world should be done only after finishing the game in order to increase longevity. Which I don't quite get, as I don't see how much more time the player would need to do those tasks after the end of the game compared doing them before the ending. To me, it just feels like an additional burden, something that takes away my enjoyment of the ending by making me run around anti-climatically after that ending. The worst offender I can think of at the moment is Arkham City, which made it impossible to collect all the Riddler trophies before the extremely beautiful ending for reasons I cannot comprehend.
To sum up my ramblings, I do not see the benefit of designing the game so that the player can not complete everything before the ending of the game.