Most heater core failures are a result of electrolysis. These leak due to pin holes caused by electricity flowing through the cooling system to ground. There are two main causes of electrolysis..... faulty engine grounds, and non-maintained coolant getting acidic causing it to become ionized and produce voltage. (like a battery.)
You can test your system with a voltmeter, place the positive lead in the coolant and the negative lead on the negative post of the battery. Any reading above .3v can cause heater core failure.
Essentially wolfox and BlazerLT are both correct.... Dexcool uses OAT (Organic Acid Technology). Higher concentrations of dexcool means higher concentrations of acid. The higher concentrations of acid cause the coolant to become ionized and "eat" or "burn" through the core.
All the above would also apply to other parts of the system, such as the radiator.