The reason that you don't want to strictly use water in its pure form is that radiators are typically made of aluminum and water blocks are usually made of cooper. It's like a battery - the resulting breakdown of the metals because of the galvanic response of having two metals that don't work well together will hasten a breakdown of your equipment.
The thing is, it should just be about what is the best thermally conductive liquid - if that was the case, nothing is better than pure water.
It doesn't matter what you add to water or what you use, nothing will do as good as pure water. But, since we know it is not adviseable that you use just water than we know that you will have to sacrifice a few degrees of cooling regardless. The thing to do is balance the characteristics of other coolants with what you consider are priorities.
Coolants and additives offer: prevention to biological buildup, lubrication, anticorrosion and nonconductivity (depending on what you get).
Personally, I use nonconductive fluids in my system (PrimoCHILL ICE) and have done so for almost 2 years. There may be times when leaks appear or spills happen. I've witnessed, first hand, how nice it is whne coolant makes contact with a powered computer part and nothing happens - because the coolant was nonconductive.
You can't put a price on peace of mind....