There are a lot of opposing opinions out there about the use of ultraviolet sterilizers (UV sterilizers) which can make it hard to decide if you would need to add one to your set-up.  Hopefully this article can help you think through if you want to use a UV sterilizer or not and make an informed decision either way. 

The first step in deciding would be to better understand what a UV sterilizer is and how it works 

UV sterilizers are a very simply designed type of a filter.  They contain a ultraviolet bulb protected by a water tight quartz sleeve. Water inters the sterilizer and passes over the sleeve becoming exposed to the UV light.  The strength of the UV bulb can range anywhere from 9 watts to 100 watts in size.  Always stick to a good brand name and follow the manufactures recommend bulb sized based on the total water volume in your set-up.  Their also two main different designs, a external style filter that requires water to by pumped through it, and a internal (or completely self contained) style were you just place the filter in your aquarium and plug it in.

UV sterilizers will never: replace biological or mechanical filtration, allow you stop completing water changes, or remove excessive nutrients (nitrates and phosphate). 

The second step would be to better understand the purpose of a UV sterilizer

You must first decide what you want to achieve with your UV sterilizer as there are two main purposes for setting up a UV sterilizer each of which better suited to different goals and each require a slightly different approach for use.  Other than having a properly sized UV bulb, not fully understanding these approaches is the most common cause of not realizing the potential benefits of a UV sterilizer. This first approach is to help control free floating algae and algae spores.  This will best be achieved by a higher flow rate through the sterilizer.  As these organisms are smaller, they need less exposure time to the UV light when passing through the sterilizer to become effected by it.  The second approach is to control free floating parasites.  As parasites are larger in size as compared to bacteria, they will require longer exposure to the UV light which can be achieved by reducing the flow rate through the UV sterilizer.  By purchasing a good brand name UV sterilizer and following the manufactured recommendations for flow rates, you will be maximizing the effectiveness of the UV sterilizer in either approach.

The third step would be understand the pros and cons of using a UV sterilizer

Based on my experiences using a UV sterilizer on one of my set-ups as well as what I have read, there can be many pros and cons with using a UV on your marine aquarium.  The below are just a few of these pros and cons:

Pros:
-Reduce / control free floating algae and algae spores
-Reduce / control free floating parasites
-Help to reduce the impact of parasite outbreaks, like ick, providing you a better opportunity to treat it.
-Help to keep the water crystal clear and/or eliminate bacteria blooms in the water.

Cons
-They will not remove all parasite as they have to be free floating in order to be effected by the UV
-They will not help with the removal of existing algae in the tank and on the rocks.
-UV sterilizers have been reported to affect some chemical compounds affecting some types of aquarium supplements being dosed.
-They can reduce the diversity of free floating microorganisms which some corals and filter feeders need
-UV filters need to be accurately set-up or they will be useless
-Extra on-going maintenance of cleaning the bulb cover and expense of replacing bulbs every 6 to 12 months
-In most situations, the same benefits of using a UV can also be achieved by other methods
-There will be limited impact on ick as they are only free floating for a smaller portion of their life span.

Whether or not you use a UV can be more of a personal preference which is also based on the exact needs of your set-up as no two marine aquariums will ever be alike.