I don't think it's necessary, it's just one way of approaching the problems associated with ventilation and moisture removal. I've had hives with solids and hives with screens, currently they all have screens. Perhaps you could constuct the hive body with a permanent screen but the capability to slide in a solid bottom if you decide to change later?
Brian D. Bray, I did something similar last year. Hive boxes sitting directly on rails of hive stands. No slatted racks, no screens, no bottoms. They did well. 24/7 there was a cluser of bees to be seen on the underside of the frames, it varied from baseball to softball sized. I actually layed there and watched them a few times, I once saw the cluster "spit" out a wax moth larvae and a bee followed it to the gound and really tore into it! What I found interesting was that the cluster of bees underneath remained as just that, they never built the nest downward, never drew any comb under those frames. I figured with the winter coming I'd better do something to baffle wind so I reinstalled bottom boards, but I intend to try it again this season and make further observations.