IMO, this strictly depends on the type of reef that you are wishing to run.
For SPS Reefs, a good quality skimmer really makes a difference. This allows you to depend less on using GFO and Carbon for nutrient removal. Also, you don't risk stripping your zooplankton from the water column.
With that being said, you can run a sps reef on a lower quality skimmer, run gfo and carbon, and bump up your water changes. It is just more effort and much more risky to run a sps reef this way. I choose a good quality skimmer for my nutrient removal.
All sps reefs need some way to supplement calcium and alkalinity on a regular basis. I choose kalk to do the majority of these supplements. On my reef(s), kalk is the most important part of keeping my acros healthy.