Nitrite and the Reef Aquarium by Randy Holmes Farley
As much as Randy Holmes Farley explains how there has to be several hundred ppm nitrite to be toxic to saltwater fish (unlike freshwater fish who are very sensitive), nitrite is still important to test for at times. If you have nitrite chances are you have or recently had some ammonia in the tank (as Randy also states), and you may have missed this with your ammonia kit. If there is nitrite showing up in your tank after the cycle has finished there may be some serious problems brewing. Nitrite is also a good indicator that something is amiss in the tank, often a more sensitive indicator than ammonia. Another thing to note is that there has been little study on the effects of nitrite toxicity on invertebrates.
I test for nitrite when cycling the tank, and at any time there is something amiss in the tank; corals fading or bleaching, fish panting, algae blooms, etc. Once the tank has matured these types of issues are rarely seen, so the warrant for a test kit later on is likely not very large. You can always get your LFS to test it for you. I do think it is a handy, and important kit to have in the first year or so of your reef's existence. I think I get my money's worth since the API kit costs less than $10.