All it takes is one time to trip over a rib, fall into the drink, and survive multiple attempts to get back into the boat to make you want a flat floor. Some boats have REALLY 'tall' ribs (like 4 1/2" tall) and without a floor on top of those, you WILL trip on them. May not be a fall-into-the-drink type trip, but at some point you will lose your balance.
Another reason-if you have closed cell floatation under the floor, you stand a better chance of recovering the boat if the plug comes out. Or like the one guy I saw a few weeks ago, gets a little out of shape crossing a shoal (well...more like rapids than a shoal), boat went under briefly. Luckily, it floats when completely swamped so they were able to recover it. I couldn't get to them as I was above the 'shoal' and they went down /over it.
If you use aluminum for the floor material over the correct thickness floatation, you can use thinner material-which is obviously lighter-and reap the benefits of more floatation, a floor that's easy to move around on, high resale, and not much of speed loss (if any). I pulled my entire floor out, along with all the floatation, and the boat was no faster and no slower. Made no difference on mine. My floor is .100" aluminum and takes up about 6 feet of the boat floor, from the rear seat to the front deck. The whole thing might be 25 lbs including floatation and rivets.