You're right, it's mostly a thumbsuck based on forum-trawling, but also comes from my own experience. In general it seems the whisky improves in the open bottle, though I have left a small enough amount in a bottle to go 'flat' and I really dislike the effect. I also notice with just a bit left in the glass after a few hours the flatness creeps in. Remnants the next morning smell completely lifeless and stale to me.
I find whisky needs to open up in the glass over 20 minutes or so. Newly opened bottles may take a bit longer, as there's been less exposure to air in the bottle. IMO this is why a whisky 'improves' in the bottle as the level drops - more air, more 'opened up'. For me, I'd rather keep what's in the bottle as close as it reasonably can be kept to what was put into the bottle in the first place, and let it open up in the glass.
Horses for courses. I freely admit this also feeds my adhd