Interesting post. Yandell discusses two things, the extent of the arm bend and which hand is more dominant. He says
1) the Williams sisters hit more bent arm and more open stance, like an off-arm forehand &
2) many of the men hit straighter arm and more closed stance and with more bottom hand dominance.
I just went and played with some shadow swings and find that I can swing from an open stance with a straight left arm (I'm right handed). So I'll need to go on a court and see what problems this creates. Because you have to reach over with the bottom hand, the shoulders can't be as open as on a single handed forehand, but I still seem to be able to generate plenty of racket head speed.
I think it may have more to do with the takeback and balance, but that's just a hypothesis.