If you have a lot of chickens in not much coop space and aren't going to be cleaning the roost poo very often, I suppose it makes sense to prevent the chickens from stomping it down and making it harder to remove (i.e the traditional type of droppings pit, which is really just a screened-off area to prevent them trompling under the roost).
However, you're doing this at the expense of giving the chickens even less floorspace (unless your mesh or slats over the poo pit are such that the chickens are not reluctant to walk on them), which to me is a major disadvantage.
To me it would be a lot better and more all-round effective to correct the fundamental two problems there, though -- don't put lots of chickens in just a little space, and remove the poo from under the roost as often as possible, ideally daily. (People get this idea that daily cleaning of a solid droppings board is laborious, time consuming, or unpleasant. I would invite you to come over to my place. If you blink you will miss it. Wide scraper to 'snowplow' poo off in one long stroke along the droppings board, into a bucket carried in the other hand, ten seconds and the pen is done)
IMHO, if you are going to clean the roost poo out fairly frequently, like daily or every few days, a solid droppings board that you scrape clean is more effective. Or, if you are not going to clean it that often, but have a good LARGE space for not so many chickens (why do people always think this means you have to build a huge coop? It doesn't. It just means maybe having fewer chickens) I am not sure that a screened-off droppings pit really offers any major advantages, as the roost poo will naturally form a 'ridge' under the roost that you can shovel out whenever you work up the energy or your nose tells you it's past time.
As far as removeable drawers/hatches/etc, I'd say their major utility is in a large reach-in coop or a very short playhouse-style coop, where it is a big convenience and labor-saver to be able to bring the poo deposits to YOU to clean, rather than you having to crawl/stoop in there to get all of them. In normal coops I think people mostly jsut build them as a "hobby", because it can be entertaining to see if you can gadget things up to absolute minimum labor input, not because they offer real big practical advantages. (I am not knocking the entertainment value of designing and building things like this, I like doing that sort of thing too, I'm just saying, there is often a difference between 'fun challenge' and 'major utility'
)
JMHO,
Pat
