Screws actually weaken the wood by disturbing the fibers. With nothing but the screws to hold the walls to the floor (or any other joint) the loads from bouncing and flex will quickly tear the screws from the wood fibers.
Now if you use an appropriate construction adhesive (or for nice tight cabinet joints, a wood glue), you get a much stronger bond because the loads are spread over a much greater area, which helps to stabilize the joint and keep relative motion from busting everything apart.
"Glue and Screw" is the mantra; just do it.
