(Link below) This compares two different failures, one bonded and the other not bonded. The top of page 6 briefly discusses the option to bond or not.

Also interesting was the summary of the ISO 9033-1:1994 standard (the EU standard that my boat should have been built to) that says: "Through-hull fittings for water shall be corrosion resistant, defined as material used for a fitting which, within a service time of five years, does not display any defect that will impair tightness, strength or function." Based on that alone, the fittings in my boat have outlived the expected lifespan by a factor of 2, and are due for preventive maintenance replacement.