A real stepside from back in the day had as much functional room as a normal bed since a normal bed had wheel wells getting in the way. Step sides were much easier to load, clean, etc. Not to mention things could be mounted outside the bed like spare tires, gas cans, and tool boxes that would take up precious bed space on a regular bed. So no real space saving going on by using a regular bed.
Step sides were designed for the workin man! You could mount all your tools on the outside and still fill the bed with gravel or cow poop and not get it stuck in everything!