Used as sub-roadbed for C-girder (steel studs on edge, act like L girders) benchwork, quarter-inch plywood is adequate since it can be reinforced to prevent sagging. Just screw a piece of angle iron to the underside between risers.
Since the angle iron usually takes the form of cutoffs from the joists, it has little cost.
Granted that it won’t support 90 kilograms of me standing on it. It doesn’t have to. All it ever supports, beside itself, is a few ounces of extruded foam and flex track, and maybe as much as a pound of train between risers. Landforms are supported on risers, and simply fastened to the subgrade. Given that, I saw no need to build benchwork like the floor of an upstairs bedroom, or a deck that would support a Mosler safe.
I actually had a sheet of 3/4 EXT-DPA plywood in the stuff I moved from Tennessee to Nevada. Shortly after arrival it had cupped into a totally useless configuration. If I had gotten creative with a skilsaw I probably could have salvaged a free-form salad bowl. These days it has weed-suppression duty in my narrow, walled side yard.
Chuck (Modeling Central Japan in September, 1964 - in the dessicated desert)