Even if you don't have to stand, sit, crawl, or walk on your layout, thin cheap plywood will warp. You don't won't your smooth track work to become uneven with dips and slopes, because it will cause derailments. It may not happen right away, but it will happen eventually. Build it strong, and heavy duty the first time, and you won't have to ever worry about it later.
Really not that much weight? There's a lot more weight than you might think, especially when you start adding scenery. 
