I have had SW tanks on and off since 1975. There were no skimmers then. Sure they are highly beneficial, but I remember the days of undergravel filters with air-stones.
IMO; Skimmers are a great accessory for a FOWLR tank, but a NECESSITY for a reef tank. This is another of those questions that will result in lots of opinions---none of them wrong;because of all the variables.
i actually dont run a protein skimmer on my 2nd reef tank. it is 72 gallon. i hv polyps and lps in it. i just feel they like the organic stuff floating around. my main tank i hv a sump running.
i dont have one in my 120. my fish and coral are doing fine. i just do frequent water changes. i am going to set one up soon though. been running about 9 months without it
If you have the means and capacity to add a PS, you will be really surprised on the gunk and waste you pull out! I dont care how many water changes you do.