We use a UV on my uncle's turtle pond (I built the pond with him last summer).
In theory I don't think it serves too much of a purpose with nanos. When there is a serious concern with bacteria, parasites, and/or algae infestation a UV can be the difference between thriving and unable to function. Small ponds - for example - with their finite amount of water and close contact with runoff, insects, animal droppings, direct sunrays, etc, rely very heavily on UVs, and depending on the climate, a UV is sometimes as necessitous as water movement.
But I dont believe they will catch on too much with our little salt tanks. Too many people are successful without them to warrant buying one (IMO).
Someone else who has bought/used one for a nano may have a different opinion, but I personally like to use as little "stuff" as possible.