I'd like to add that for those of us that don't buy into the CO2 blanket fable
, particularly over the course of time of a primary and secondary fementation which gives plenty of time for oxygen to diffuse down through an open airlock, liquid in an airlock does allow evolving CO2 to dilute and push out oxygen from the fementer.
Edit: During primary I imagine this would be like an open fermentation but with better protection. But what about after primary is complete? Do folks who use open fermentation leave their beer to age in open fermenters? If not, then my comment above would really apply to the time spent after primary fermentation.