Well if you take lessons from freshwater, you will see why skimming is a good idea. Is it essential? the answer is certainly not but it depends on your situation.
If you have access to lots of "suitable" sea water then a skimmer is not just unnecessary, but probably counter productive (because you remove protiens as well as some of the essential element specially in reef tanks which you have to constantly add manually). In this case all what you need to do is change water. A friend of a friend has a 250G plastic tank which he fills with sea water using a pump and plums it directly into the aquarium for constant water change over a couple of months.
For the majority who don't have the luxury of having sea water freely at thier disposal, changing the equivalent amount of water would be an expensive excercise and the money spent on a good skimmer is well worth it...