I would say it's neither bad nor needed, it's more of a choice. One potential benefit is a water clarifier, the UV will break down yellowing compounds (much like ozone will as well) and you'll get a "cleaner looking" water than just carbon alone.
If it's big enough you can use it as an antipathogenic agent, but you need a lot of wattage to for it to be effective this way.
Pitfalls - some people think that it is detrimental to plankton and I'm sure it is for those unlucky enough to get sucked into it but that's not going to be the majority. Also the lamp degrades just the same as a T5 or a halide or whatever and needs to be replaced every few thousand hours just the same, which can be an added expense.
I ran a 55w UV on my old tank, and although it could be coincidence, I did have a fish with ich overcome his ich after the UV was put on. I don't run it anymore but I've kept it in case I ever need to run a QT or hospital tank where it could be of benefit.
HTH
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-- Tony
My next hobby will be flooding my basement while repeatedly banging my head against a brick wall and tearing up $100 bills. Whee!
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