Plants need CO2 only when the lights are ON. When the lights are OFF they only use O2 for respiration, thus they also consume O2 from the water. Injecting CO2 at night would only make sense if you want to have a constant pH.
When most people say introduce O2 they really mean increase water surface agitation. Even airstones are mainly doing this. If you increase surface agitation you are doing 2 things increasing O2 levels a little and reducing CO2 levels. Is it necessary? I have not seen any major improvements by doing it. A clear advantage of not doing it is that some CO2 is left from one day to the other and you need less time to get to your CO2 target, less CO2 waste.
Suggested advantages of increasing water surface agitation: provide fish a rest from the high CO2 levels and suitable O2 levels, provide bacteria in the filter and substrate good oxygenation.
I have yet to see fish gasping at night because plants took their oxygen. If your bacteria population needs high oxygen levels by night, it needs them by day. If you do not provide suitable oxygen levels to them by day they will die or not do their job. To me it makes more sense to keep good O2 levels throughout the day. Sure you might waste some CO2 through degassing but it keeps fish and bacteria happy.
On hiatus till later this year