I used a 9watt on my 29g for about the first 6 months to help keep the fish healthy and kill free floating ick and other algae and bacteria...
problem is that its worthless if you have no fish in your reef...next if its too small is worthless since it wont make a dent...next the flow rate must be perfect..too fast and it does nothing...
I will probably get another one for my 60g soon, but i didn't have the cash....the protein skimmer and LR were way more important in my opinion...
you really dont need it, especially in a reef the uv kills a lot of bacteria that is useful for a reef, I would only reccomend to use one if you are keeping very delicate fish likes rays, sharks and others.