Actually, it's quite the opposite. The myth that UV sterilizers can be harmful is spread around, but there's no truth to it. It mostly stems from a lack of understanding on what they do and how they work.
UV Sterilizer can do no harm.
They are great for algae control and dealing with water-borne parasites. Green Water, Bacteria Bloom, Ich...no worries if you have a UV. I also wouldn't ever have any high end fish without one. It'd be a shame to lose some Altums or Stendker Discus to a parasite that could have been killed off by a UV.
It's not a necessity in FW, but it's a great convenience. If you have the opportunity to pick one up cheap, you'll really appreciate it.