Just for the record, I'm not suggesting that the 4 cent (or 9 cent) cost makes it worthwhile - I just got tired of reading wild speculation about the costs, and decided to do a little math and come up with a better answer. You're right about the 2 segments per day - short-haul flights do more, but when those crash it's over land, so they tend to be found pretty quickly. Only those flying over water would benefit from this system.
And I do think it's fair (and helpful) to bring it down to a per-passenger level. Sure, the airlines operate at a large scale, so any fleet-wide investment will cost zillions, and any fleet-wide savings will save zillions. But that's compared to overall costs in the mega-zillions, so the numbers are almost meaningless to most people. Suppose someone wanted to eliminate the padding on the seats, and just have you sit on bare metal, and quoted a large dollar figure savings - the first thing I'd do is estimate the per-passenger savings: if it's $50-$100 per pax, then it could make a big difference in ticket prices (PLEASE let's not go off on a tangent about how the greedy bastards at the airline would just keep the difference!) ... if it's $0.05 per pax, then no, even the meager comfort of the standard seat cushion is worth a nickle to me (besides, your metal seat couldn't be used as a flotation device)