Is it suitable for beer "bottling" or would the pressure tear it apart? If it's suitable it could be really useful since it can contain a large quantity of liquid with low package weight.
If you're referring to this bag-in-box, then there's nothing wrong with it. You basically store flat beer in a box (which is very similar to how soda fountains work), then carbonate it as you serve. You still, however, have to have a CO2 tank where you're planning to serve it, which is still heavy. Your use and shelf-life would be similar to a keg.
If you are wanting to actually carbonate beer in and serve it strait out of the bag, I think the pressure would definitely tear it apart. However if the box was sturdy enough (i.e. not made of cardboard) and smaller than the volume of the bag it could hold up and prevent the bag from popping. The problem with this setup is once you start drinking the beer begins to go flat, similar to a 2 liter of soda.
EDIT: Have you checked out the Party Pig? After looking into it a bit, it looks like this is exactly what we've been describing. It however has a disposable pouch that looks like it sets outside of your beer pouch and produces pressure to prevent your beer from going flat. I'd be interested to hear from anyone who has used one of those.
You may be referring to these cubitainers http://www.bestcontainers.com/hedpak---cubitainer-cubitainer--combination-packaging-knocked-down-cubitainer---inserts.html most lhbs carry them in 1 and 5 gallon. I've used them in the 1 gallon format for a cheap "cask" alternative. I say cask in the fact that they are carbed to low volumes (around 1.5) and that I do not pour through an engine...i instead dispense with gravity.
they're airtight and the bag collapses as it pours....so minimal oxygen exposure unless your purposefully let it in.