My line of thought is that:
1) Giving precautionary O2 (for ascent rate/deco violations) may be enough to prevent a hit.
2) If the precautionary O2 masks the symptoms of a hit, then it is also delaying the onset of that hit.
3) If the hit is going to happen, it will be evident when the diver comes off the O2 after the precautionary period (10/15/20 mins?).
4) It's better for the diver if the onset of the hit is delayed until the boat is nearer shore etc.
5) If a hit occurs, the diver will be given O2 anyway... which will again mask those symptoms.
This assumes that:
1) Once given precautionary O2 for a 'bad' dive, the diver would not be allowed to re-enter the water for subsequent dives, regardless of the absence of symptoms.
2) The diver will be monitored and assessed for DCS before, during and after the administration of precautionary O2.
3) The diver will be immediately evacuated to medical care should symptoms arise.