I've used skimmer's for a long time, and have done several tests on startup tanks. In my experience you can go without a skimmer if you have fuge (or other significant nutrient export method) UNLESS you are running a predator or a heavily stocked tank. In that case, if you want to even consider keeping SPS and most LPS happy, a skimmer is an absolute must. Most tanks I see these days are on the moderate-heavy stocked side of things, generally requiring a skimmer. On my last build I cut a hole in the bottom of my stand, and have my sump sitting on the floor, all to accommodate a skimmer.
There is new tech like filter rollers that are bleh and cause as much pain as they do good, and there are skimmers which have sort of received a standard in the hobby for 40 or so years.
In some cases, in a very lightly stocked tank one would opt out of a skimmer as a means to keep a little of that dirty (1-5 nitrate) water around. Light stock would be a few inverts and 2-3 small fish in a 40g.
