To use or not use a protien skimmer really depends upon the stocking level and size of your tank. A large number of fish in a small tank is going to produce a large amount of organic waste, in which case you most certainly would want a protien skimmer, especially if a person doesn't intend to be anal-retentive about water chaning at least once or twice a week. If the person is using tap water, than a protien skimmer might also be necessary since tap water also contains dissolved organics that could over-enrich the water in a marine tank.
On the same coin, I have also had tanks without protien skimmers and they have been just fine. I like to keep them understocked though, and these tanks have usually been ones that I've been able to refresh with natural sea water, which in and of itself, contains little dissolved organics because it is removed with wind and wave action (what a protien skimmer mimics).
~~Colesea