Having the bench flat aids in all sorts of things. Your vise jaws are in plane, you don't pull things out of square when clamping to it and so forth. How flat is flat for a workbench? That question will get some different answers. My construction grade KD fir trim around my 4 layer MDF top contiued to shrink despite being kiln dried and setting for over 2 months, stickered in my shop. This actually squeezed the outer edge of the top and it now "drops" almost 1/16" at some places starting about 2" in from the edge. This really bugs me and I have to watch for it but the top is about 30" x 84" so I have lots of flat area. It does not bug me enough (yet) to build or re-size the top as a rebuild is more likely due to dog hole positions and so forth. I heave dealt with it for years but, will get that feeling you get when you finally get the splinter out when I go to a new top. YMMV.
Any sufficiently advanced technology is indistinguishable from magic.
- Arthur C. Clarke