Alley cropping has a lot of advantages and scales well depending on one's goals. If you wanted to have large combines, you can simply make the alleys wider. Its all about tradeoffs. I'll have to look into Kentucky coffeetree, I imagine that their bean pods would make good livestock fodder.

Regarding the alfalfa wheat intercrop, the goal is actually to harvest the wheat for grain, with the alfalfa functioning as a sort of fertilizer crop (that's still harvested for forage after the wheat is harvested). There are a lot of barriers, so it's an active area of research, but it's a really interesting topic.