With backless boosters being so cheap and easy to move/stow, why would you buy a big ol' thing like this? What if one kid needs to ride home with Grandma or go with friend?
Because some vehicles are very difficult to fit 3 across! I could see using something like this in the 3rd row of my Ody. That bench (which has head support!) is too narrow for separate child restraints. The same was true in the 3rd row of my Pilot. Fortunately, I don't have to fit 5 kid in the back, so it's not an issue, but what if I were pregnant with TWINS? Then my choice becomes finding a way to fit 3 kids on the back bench or putting one kid in the front seat.
Obviously, you would need spare seat(s) for the times when your child rides in another vehicle, but that's not exactly a novel idea.
I'm not getting these "scary" crash dynamics that so many posters seem to be envisioning, at least not with the booster-only version. Boosters were described to me as "telephone book technology." That is, their primary purpose is simply to lift the child up so the seatbelt fits him/her properly. This product would do that. I don't see how it would be better or worse than 3 separate backless boosters in a side impact.
I also don't see how having a heavy child sitting next to a light child could cause problems, as each child would be seatbelted separately, and the armrests would help hold the seat itself in place along with the child. Only taking 1-2 children in the seat? Wouldn't you just buckle the other seatbelt(s) anyway?
It's obviously a niche product. It's obviously not the best seat for every situation (no seat is!). I don't know if the demand would be enough to be profitable. But, the concept itself isn't a bad idea, nor is it one for which I can see no practical use.