Reasons to use antifreeze:
1. It is an excellent anti-growth additive. After running my system for 2+ years without a fluid change with the case open (therefore admitting light) I have absolutely no growth whatsover in my system.
2. It is an excellent corrostion inhibitor. Likewise, after 2 years, my system (clear block top allows for good observation) is a shiny as the day it was new.
3. It is an excellent lubricant for your pump. My Hydor L30 is still nice and quiet and has never been plagued with the start up problems others have reported.
4. In small amounts sufficient to accomplish all of the above, antifreeze has a negligible effect on cooling, as the viscosity increase in very small.
5. Anti-freeze will not cloud or otherwise obscure clear tubing like some additives.
I'll second Busmasher's recommendation for propylene glycol; No need to put toxic material in your comp.