It's also for your safety, your body can handle a lot more G-force such as a sudden stop if you are sitting straight up. To the guy that was peeved that the FA told him to sit up instead of slumping over into the seat next to him, the same thing applies. Your spine would be a lot more likely to suffer serious injury if you are reclining or leaning off to the side. The FA may not even know why she has to ask you this and you obviously don't know but that doesn't make any difference, it's for your own good.
Nonsense, you are far more likely to survive an impact if you are unconscious verses tense and rigid.