Reading in the tub/shower doesn't involve a lot of activity that could cause the bags to spontaneously fly apart. It's not like I'm suggesting this method for water-skiing. Seriously though, what could go wrong? Any holes in the bags large enough to let in large amounts of water are immediately obvious. Tiny leaks have to get through two layers of plastic, and would take a long time (longer than your bath or shower) to allow much water through.

Furthermore, electronics aren't as sensitive to water as people think they are. Most of the important stuff is protected pretty well by the case. You could swipe your reader under the faucet, dry it off, and be none the worse for wear (though I wouldn't exactly recommend it). Barring something catastrophic, this is WAY more water than would get near your reader with the plastic bag method. If a couple drops of moisture happen to somehow accumulate inside the bags, they're unlikely to cause disaster.