Mike, we used to do this years ago when we had a university mountain biking club . The downside is that any water that DOES get in, can't get out again . This is why lizard skins are better, you can whip 'em off after a ride to let it all dry out a bit.

Another think we used to do to protect chainstays from chainwhip on very rough terrain is to cut a slice from an old tube the other way so you get a long narrow strip and wrap it along the chainstay, securing it with zip ties and each end. Again, I would'nt recommend this in wet climates.