Any system that is small enough to do large water changes to address parameter issues can do away with a skimmer. I stick with the 'under 30' rule for this. It is pretty easy to do 50% water changes in that range.
That said, one isn't always necessary, though extremely helpful. The pump for the skimmer on my 55 reef died and it took over a month to replace. The tank didn't crash and burn, but you could see the difference. I still have cyano
in my sandbed that will take some time to be totally gone once things are back in check, if I ever get there.