The dividers are on the tree to prevent people in the full tree classes (the classes where the tree is staggered, one lane starts before the other) from getting two shots at the tree. If you can see both lane's yellows, the faster car is at a clear advantage. They can release their button on the other guy's light, and then on their light, and the smarter boxes can choose between the two reaction times, leaving on the faster of them. The faster car already has a strategic advantage at the end of the track (they are the chaser, not the chased), the dividers prevent them from getting this additional advantage. This is also why you'd have to be careful about putting additional lights in other places as they could just key off of those.
Also, the size and specifically height of the tree is something not to be tinkered with lightly. Many vehicles (notably Pro Stock Cars and Funny Cars) have very limited viewing capabilities, especially around intakes, and the tree needs to be clearly visible. That's one of the reasons why the new staging lights are so preferred, as they put two functions in the physical space where one light used to be.
