Good question. IME, 'faux skimmers' clean up the surface of the water, which I do think is important. Most of this surface scum will go away with a well maintained aquarium (avoiding contact with dirty pits) and increased water circulation and surface agitation. Of course floating something like Crystalwort on the surface would probably work too.
The Duetto increases water circulation and surface agitation, mixing surface water with O2 to help break down the film (I think) and any oils/protein go back into the tank eventually to be filtered through the main filter (at least more likely to when the oil no longer sits on the water surface). You can even add a spray bar. The sponge filter can be left in it, but I didn't find it made any difference other then needing to be cleaned more frequently.
The SubCurrent Internal Wet/Dry Filter is bigger then the Duetto and I was not pleased with it. Honestly, I gave it away, I can not remember why. I think it was just too noisy for me.
The aquarium surface skimmer attaches to the intake of a power filter. So, it does run through the filter and add O2, but it is much bigger in reality then in the picture. I also found mine to be finicky and hard to connect, depending on the filter size. At that time I was using an Eheim ECCO Canister 2232. So, what filter it is attached to is a consideration.