Given that over 70% of air crashes are survivable, why on earth would you want to give your passengers a position that is designed to kill them? There has been extensive scientific research into the best position to survive a crash with the least amount of injuries. The brace position in economy when the space is too tight to put your head on your knees is detailed in the safety briefing - you do watch the briefing, don't you?
Proven fact - watching the briefing and acting on the advice improves your survival no end! The brace position is also completely different in Club and First where the seating is rear-facing. The only people on the aircraft who have no designated brace position are babies and pilots
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So, all reasons for bracing that don't involve best-outcome survival are disproved here and here. I could put up links to AAIB stuff too, but it's a pita to find, and Snopes and Wiki quote the best references, which are paper-only journals.
God, I've gone all boring on the subject. Having said that, I do get het up about aviation safety. Which is something I'm sure you'll all be very pleased to know. 
Zippolighter get off the naughty step! The sad fact is that even if you moderate your carry-on weight, some other fucker will try and get all their worldly goods into the locker above your head. Barstewards. Everyone should travel naked and sedated, with all their stuff in the hold, and while they are out of it have subliminal messages played into their heads that no one knows how Ryanair can sell flights for 50p on a jet which cost 60 million squid and needs 12 tons of fuel to get you to your destination. It would make my life a lot easier.
BTW I don't know how to get a baby to sleep all night - I've never managed it yet. I suspect it has something to do with not having enough bedrooms in my case. (The extension is ready for it's roof though. I don't know if I can contain my excitement!)