As far as I know, the reason for not using the same bottles for water as for coke is, that the material of the reusable coke bottles gives off a tiny amount of aroma stuff - which might be noticed in pure water, but is indetectable in coke or lemonade...

The material of the one-use water bottle does not give off these aromas, but is not stable enough for reuse (under industrial conditions, where the bottles are moved around by robots...). Of course, if you handle such a bottle carefully, it can last for years.