My understanding was that the floors were open because waterproof zippers are so expensive! They are expensive, and they break and leak. Nowadays AIRE will upgrade to a Sealed Floor Pocket so they must have figured out a way to do it well, but it is still more money. I believe the whole "water ballast" theory was a result of marketing. I mean who really likes the thought of water in their raft?
That said, I think there is truth to the water ballast concept. In fact, I've decided that a Super Puma at 5'9'' wide might be just as stable as a SOTAR ST at 6' or 6'2" and will fit down tighter channels. So while I love SOTAR, I am currently in the market for a used Super Puma. I hope I'm right
I've been in a lot of boats over the years and they are all stable to some degree until you're upside down. Avons always seem to feel the most stable. But, there are many reasons for owning a raft, and I bought my 14' Hyside because it was 30lbs lighter than an Avon, was faster, could be rode hard and put away wet, could be cleaned up and rolled up for years and years with no real damage, etc.