I've noticed my pH has been less stable since I removed my skimmer - no other changes really. I imagine my aeration is poorer thus more CO2 in the water column.Just for the record. I agree with taricha that I’d run a skimmer even if it accomplished nothing but aeration.
I don’t consider skimmers any great way to control nutrients, but they do help remove organics of various sorts that can be toxins, yellowing compounds, bacteria, and suspended particulates.
Stable pH ain't no scam.