The long story short so I don't have to go in depth on this, don't bother. Unless ALL the return water is going through a UV sterilizer and the sterilizer has enough power to be able to kill parasites at your particular return flow rate its pretty much a waste. By waste I'm pertaining to not being as effective as it could be. Better off leaving it to a quarantine set up or for pond clarifying of single celled algae. HTH
These two comments pretty much sum up my thoughts on this as well. I don't see any reason to include one in a reef set-up, and it kills the good just as well as it kills the bad, so there is a reason not to include one. Proper quarantine procedures negate any possible need for a UV unit on your display system IMO, and in most instances, UV is only marginally effective (if effective at all) with marine fish issues anyway.
It's like a air purification system in a house.It's nice to have it,but not really needed.
Personally,I wouldn't run a UV unless I needed it.And that's only happened once a long while back when I had a green water bloom in a tank I was running hypo salinity on.
I have been running one for the past couple years and love it. Would run it again on a new system if I had one lying around, not sure if I would go out of my way to buy one though. I don't use it to try to kill parasites or anything like that. My main use is to kill free floating algae. It does a tremendous job at water clarification. I run about 12 hours a day slightly delayed from my lighting along with carbon 24/7.