Thanks 5sgte. I had a look at those references and some other  references on brake fluid being hygroscopic. I had the same thought when someone asked "why use a fluid that is hygroscopic ?" (absorbs moisture). The explanation is that the absorbed water is distributed throughout the brake fluid and does not affect the fluid's boiling point. The worse alternative, a fluid that rejected moisture, would cause the moisture to pool and boil at 100c. 
 Nulon suggest replacing their Dot3 every 2yrs.