i've had tanks with and without UV sterilizers and all i can say is this.
If you use the UV sterilizer appropriately, with the right size and right flow, it will help keep the numbers of harmful bacteria and other microbes/viruses down, but logically theres no way for it to discriminate against any particular organism so it kills good bacterium as well.
With Ich in particular, i noticed that the parasite count decreased with the use of a 30 watt UV sterilizer on a 125 reef tank with about 500-600GPH flow on it. I made sure to talk to john with the proper set up for it and it seems to be doing its job. Also make sure you replace the bulb every 6 months to make it as efficient as possible.
The key point is that UV sterilizers help, but aren't a remedy to all diseases int he hobby, i would still QT (ever though i dont

), and use other forms of medications when signs and symptoms appear on your fish.
Also, i've noticed that using a UV sterilizer and active carbon helps make the water more clearer.