Joe, I think the flotation in the larger (18-22' models) boats is more to guard agianst sinking if the hull were damaged and they took on water. Water seeks it's own level, so I doubt that your entire cabin volume would fill if adequate flotation were fitted, and enough air would be trapped inside to at least slow down the sinking long enough to be rescued in a coastal mishap.
O'DAY used to warn clearly in their sales literature that thay provide closeable cabins for just the resons you give, and smart sailors close the hatches in rough conditions. A capsize (not likely, but not impossible o na 222) is another cause for concern, however........ it that case I'd be more concerned about a cockpit locker opening and flooding hte boat than the cabin hatches. Boats like the O'DAY cruisers (192, 20, 22, 222, 23, 25, 26) will actually float high on their sides if knocked down, limiting flooding.
For coastal cruising (like on Mount Hope Bay or Narragansett Bay or Buzzards Bay) I too would go without flotation in a larger boat than my DS II to gain stowage room. But if I were sailing offshore at all (like to Bermuda?) I think I'd want some kind of flotation, even if it were an inflatable tube setup. Even my DS II, if not holed, could safely sail without flotation as originally built. HOWEVER, I REALLY like the comfort factor of having some styrofoam in my bilges, especially since I have several access hatches cut into the cuddy floor, and in a good rainstorm...... my bilge will flood.
Again, my personal choice........... I would not advocate others removing flotation without full consideration of their sailing risk level. Full flotation DOES create a built-in lifeboat if the hull were ever holed.
PS: Our old CAL 21 that we owned for 38 years did not have any flotation installed. Never worried about it, except when we developed a crack in the hull at the forward end of the trunk for the retractable keel. It was a bit disconcerting the time that I went aboard and found the floorboards in the cabin awash.... however, on that boat there was only about 3" between inside of hull and top of the floorboards. Still, it was a little unnerving until I bailed out the water and found that it didn't rise again noticably.