Engine goes... KNOCK!

I won't say you have to run anti-freeze no matter what, its your bucks and your engine.

But Anti-freeze also has other additives that conditions the cooling water so it does not become to acidic thus preventing your engine from being oxidized from the inside out. Its even more important when you have aluminum parts such as water pumps and heads bolted to cast iron blocks. Just my 2 cents on the pros and cons of anti-freeze 8)