Some run skimmers some dont, its just like carbon, supplements and water changes. As mentioned a skimmers main purpose is to remove undissolved organics from the water before they lead to nutrient problems, that includes planktonic life which exists in most healthy reef tanks. Some skimmers can skim a whole lot more than others, some do nothing more than aerate the water
Nutrient import/export must be balanced or you will see one of two things, starvation, too little nutrients, or excess algae and elevated nitrate concentrations due to excess nutrients. IMO a skimmer is a really effective piece of equipment, I do however run a tank without one, but only because the tank seems to have reached a balance without one, of course there are several other methods of nutrient export in place such as a DSB, activated carbon, macro algae growth, and to a point water changes
And the skimmer is not far away if I feel its ever needed. Theres a saying that if you know the rules you are allowed to bend them or break them, but first you have to learn the rules, I would suggest a skimmer to all new reefers, its somewhat of a safety net when its comes to nutrient control. HTH
