You can't bench your finished game before it exists and you don't really want to rock the boat late in dev, either.

It was a fundamentally sound default that they revisited. Then they blogged about the relatively surprising difference it happen to make in their particular game. As it turns out the loading is CPU bound anyway, so while the setting is doing it's job, in the context of the final game, it happens to not be the bottle neck.

There's also the movement away from HDD and disc drives in the player base to make that the case as well.