In facilities which I cannot put ,or grow, plants in, I definitely do. Especially in lab settings where the fish are crowded and overgrowth issues exist - and also, when plants are dormant but fish and beneficial bacteria are not. A veggie filter can reduce the need for UV but sometimes, there just are no plants. SEARCHTERM: Philodina - But The reason I run my UV year round is because with variations in pond temperature there is always the risk of a bacterial or a green water "bloom" which is a sudden and exponential proliferation of algae or bacteria. These blooms can change the characteristics of water rapidly.
Bacterial blooms are bad for the fish because the fish don't know if the bacteria which are proliferating are "good" or "bad" and the response is the same strain on the fish: They develop what are known as "Reactive macrophages" in the gills - the bodies immune response to high bacterial counts. In most instances the bacteria are harmless and the fish are unaffected, but the stress of mounting this immune response eventually takes it's toll.
Green water is NOT BAD for the fish, in fact it's good for the fish but I didn't spend three thousand dollars on my pond to look at a vat of pea soup.
If you turn off your UV during the winter, make sure you have it back on again by the time the water returns to the low fifties or you will be behind the curve on a greenwater episode for Spring.