When I built my Roubo bench several years back, I added the customary shelf between the stretchers. I mostly use this area for clamp storage more than tools. For whatever reason, it also tends to also attract scraps of wood, unused tools and bits of debris. About once a year I go thru the mess and clean it out.
My Moravian bench does not have a shelf, nor have I ever really missed it not having one. Today while cleaning the pile of accumulated junk from under the Roubo (again), I was thinking that maybe a shelf is more trouble than it is worth.
— Will Myers