I don't like the concept personally. Just one more moving part that gets wear and tear and eventually jams. I would rather just toss in way too much litter, let them poop on it, mix it up and then rake it out like 2-3 times a year.
It also heavily depends on how the tray slot is made - if it can be pulled out while able to keep a good deal of material on top of it, I can see use in there. But if there is an above board that limits how deep the material can be on top of the tray, this complicates things.
But yea, a poop tray under the roost definitely has value too. Most of the coop poop will happen while the bird is sleeping on the roost, so it ends up in a very predictable spot. Having a quick-removal panel that you can scrape off would keep the litter decently clean. The trouble here is that the poop tray does nothing to mitigate the smell, so has to be cleaned very often (daily, I hear).