Can't see why people would buy innerbags.
1) They are most of the time expensive.
2) They take a certain volume from your case, which means less luggage.
For several years now I use plastic bags to keep the water out, and two years ago I bought two waterproof bags, you know those used by most adventurous guys. Advantage is that they are waterproof, and you can compress them by pushing the air out. Think I paid around €15 each, and they do their job.