Sad to say, having a droppings board does NOT prevent there from being plenty of poo in the floor litter to happily compost (to whatever degree your conditions permit -- a lot of peoples' setups are not *going* to get in-house composting *anyhow*). Plus which, in a typical backyard coop situation even if you DO get composting going on you are generally NOT getting a real big heat boost from it anyhow so even if you have somewhat less poo doing somewhat less composting, who cares?
And removing the poo practically right away (first thing in the morning), by using a droppings board to intercept it, does a LOT to help air quality.
So that, to me, is the rationale for using both.
But again, to emphasize what ddawn says, there IS NO SUCH THING as "the" deep litter method. There are a whole buncha different ways you can juggle things, for all different situations and all different purposes. Pick whatever methods work happily for you, and don't worry about what other people may be doing because it may suit their situations differently than yours
(not the o.p. specifically, of course, I just mean the generic "you")
Good luck, have fun,
Pat