apisanif,
Do you get the point raised by Opel? Very well explained indeed and that using battery water will only raise your engine temperature as its fails to cool down sufficiently. While i don't think your engine will blow if you ever decide to use battery water inside the radiator but certainly its too hot. Would you rather run your engine hot or slightly cooler?
The whole idea about using coolant is the ability to resist rust and most importantly increase the boiling point of water. Every car should use it. Also, a plus point about using coolant is that coolants normally come in very fancy colors ie. fluoroscent green, light blue, bright red and this is a very good leak indicator as compared to normal water because you might dismiss it as aircon water if there's leak.