In my experience, a floating plate still reduces flex over no plate at all and a fixed plate further reduces flex. Also, if you accidentally step on your keyboard, a fixed plate design will transfer the force to the case via the plate instead of the PCB.
I note that PCB mount designs tend to have stand-offs more evenly distributed. Classic Cherry keyboards have bars that extend along the length of the keyboard and more modern keyboards usually have a stand-off or two somewhere in the middle. Fixed plate designs, OTOH, are almost always fixed via the edges with zero support in the center area. Intuitively, I think edge-fixing with a stainless steel plate is more than adequate to prevent noticeable flex even on full-length boards.