I think I have an answer. While the idea would be great, the reason you don't see this done is because if someone loses something on a coaster it can hit someone below.Someone tell me again why they couldn't have woven this and Slinky Dog Dash together on the same footprint in Toy Story Land?
This is why the attractions being placed inside the helix of California Screamin have covered queues and rides. It is also why Dualing Dragons stopped dualing. And they have that ugly net hanging off of the Hulk Coaster.
They need to make the raceway an indoor attraction. Creates a greater sense of speed and easier to electrify. See Andretti carts.