I can see the balance reasons, especially because it looks like the Aliens can kill marines in less than a second up-close. A stamina thing that prevents multiple swings like L4D would be a possible solution.
I read in one of the interviews, maybe from Pitchford, that he said the game was technically still in pre-alpha? They're also quite unsteady with their talk of release dates, but at this point I'm all for waiting longer as long as they continue to improve and polish the game.
It sucks having so many great games rushed out the door. Nice to see a company take it's time, even though the circumstances are quite volatile with this particular title. It's so hard to wait, but as long as the end product is a better game, most people would probably prefer that.