A couple of summers ago a Brit girl cycling in France in the summer killed herself by drinking only water. When you sweat you sweat out salts, you need those salts, if you drink just water then you encourage the body to sweat out more salts. Of course, if you're eating right and only working out for a couple of hours in normal conditions it's no big deal either way. I used to be able to wring a large puddle out of my shirt half way through class and drank only water to no ill effect, but if you're really training hard for long periods sports drinks containing salts are the way to go. Sugar and absorbtion and so on... most of us don't train at a level where it matters.
http://www.guardian.co.uk/weekend/st...492713,00.html