To my mind, if you use U.V. to sterilize the water the fish's immune system never gets a chance to strengthen. Then if it (the UV
lamp) fails or becomes ineffecient with age there might be the potential that your fish will be weak. Then they may succumb to a normally reatively easy to treat illness.
Immune systems are meant to be exposed to a mild level of nasties. Keeping a good tank maintainence schedule and strict quarantine should really enough to have healthy vigourous fish.
These are just my thoughs and others may disagree. I wouldd be interested to hear what others think too.