I'm with Arkroyal on this. If you're going to insist on looking logically at security then you have to do the same with lifejackets. There are many devices that you could put on a plane for the same weight / cost as all the lifejackets and which would save significantly more lives. Lifejackets are an historical anachronism from the days of flying boats which (as far as I know) have never contributed to saving anyone in a big passenger jet crash - get rid of them!