I don't have a protien skimmer on my tank and it is doing just fine. Skimmers are not like water, you don't technically HAVE to have one. They just let you get away with putting a heavier bio load in your tank, but if you keep the load down your tank will be fine without one. Don't get me wrong, I will most likely get one eventually, but the question was if it was ESSENTIAL. As all accomplished aquarists know (and heaven knows I am not one of them,) there is no right or wrong way to do a reef tank, just the way that works for the person who is maintaining it.
P.S. Some animals, especially clams, do well without a protein skimmer as it will remove the suspended organics that the clam feeds on.