Here's the skinny. Plastic can contribute an off taste to your brew. That's the biggest reason why not to use it. I do know people who start their cider in the carton, but it's just not the best idea. A far better alternative is to use a 1g glass jug of apple juice, Whole Foods sells them for like 8$. You'll need an airlock and a drilled stopper, but you can improvise with a baloon carefully strapped to the top of the container with a hole poked in the tip ;D basically you need to let co2 out, and keep fresh air and germs outside. Keep in mind that cider generally needs to age for a year, so it's better (and really, not much more expensive) to do it right.