To be completely honest - distilled water isn't all that much better than tap water.
Yes - it does get rid of the Na, Ca, Cl (etc etc) and any other cations and anions that are dissolved. However, it does not get rid of the H+ in the water. Since the H+ are still there, the water is still slightly acidic, and thus (after time) can eat away at any metal. Water is one of the best solvents (scary, eh?), in general terms. So in effect after awhile (even if using Distilled H2O) the H+ will 'eat' away at some of the metal and thus you'll have ions again.
Waterwetter (et. al) helps rid the free H+ in sol'n (I'll bring my bottle in to school and test the pH tomorrow). If i'm correct in my thinking, it acts as a buffer, to limit the pH range. It also does prevent corrosion - likely by bonding with those free ions (listed above), to keep them from affecting the metal in the system. That's why the label says that when used with distilled water - it will reduce scaleling - while with tap water, will just prevent it.
I could break out some reducing reaction equations...wait - I forgot that after I passed chem
Oh, and when I was setting up my water cooler - I was still getting oils - and I flushed the system about three times with nice hot soapy water.
I guess - without going into a control lab using precision equipment, there will always be issuses with corrosion. All we can do is have enough foresight to take precautions to limit it.
anyways
cheers!