That's what I mean, perhaps the bottles matter even for one person because Ripley (or whoever) might have to wake up and operate the ship while conscious at some point, but then Ripley bails on that plan because she doesn't really have a choice and rolling the dice in the shuttle (or even eventually suffocating because of no air) is better than whatever the Alien might do to her.
I'm just tossing around retroactive explanations in light of the knowledge that there's a third bed. Graham might chime in with his own thoughts; ultimately I'm pretty okay with the existence of a third bed.