For what it's worth, I recently got to test out the "plastic breaking down" theory.
I work in a food warehouse, and about 6 months ago I put a half pallet of water up in a rack where nobody would disturb it.
It was a watermelon doro bin filled with nestle purelife, poland spring and my store brand loose bottles of water.
There it sat, in 38 degree consistent temperature, no sunlight, for 6 months, until yesterday when I pulled it down and some of my night crew sampled it.
The overwhelming consensus was that the water tasted off, stale, and had a funny aftertaste. I personally tried 6 bottles or so ... all of which had a chemical aftertaste.
Is it still drinkable? I hope so, but this test has confirmed in my mind that storing water long term in cheap plastic bottles isn't ideal. It most definitely picks up a taste, which implies that it has also picked up a chemical or some sort. End of the world? No, but not what I want to deal with.
ETA - the "expiration" date on the water I remembered to look at was sometime in 2012.