It depends.
First - UV is no defense against ich. Don't think UV will either cure or prevent ich. It can kill the trophants - but in practice it won't kill enough to make a difference. By the same token, UV is not going to wipe out your pod population. It may impact the reproduction of broadcast spawners, but probably no more so than other forms of filtration.
UV does little on a clean system. UV striking organics in the water produces oxygen radicals. If there are few or no organics in the water few radicals are produced - so on clean water UV essentially does nothing but burn power and add a little heat to the tank - but doesn't hurt anything either.
In systems which are not heavily skimmed and have high levels of organics in the water - UV can be very effective, helping to prevent bacterial blooms and breaking down humeric acids (yellow water).
I use UV in conjunction with carbon, in my (typically smaller) systems when I have insufficient or no skimmer, or am using only a canister for filtration - as these systems tend to be fragile in the sense that they are quicker to foul from over-feedings etc - and have high organic content in the water.
I don't in systems which are heavily skimmed (typically larger) - where it is not as likely to encounter rapid organic build ups in the water column.
In no case does UV hurt your system. Its just more effective in some situations than others.
