I somewhat disagree, but assume that you are correct. So what? On the gaming night in question - given how the game progressed - the most important thing that happened all night was the player intervening to help move the story forward.
So I see the issue, you are defining importance as "possessing the most authority. No one disagrees that the DM has the most authority. Instead, I counter that importance is based on what is actually done - not what one is empowered to do but remains undone.
Actually, I've mostly avoided using the term "important" in this thread because it's such a subjective term. Even in this case. You're saying that this player was the most important because he progressed the story. Fair enough, I accept that. Your initial post actually made it sound like the player was derailing the game, hogging the spotlight for a half hour to play pranks, and that you were somehow powerless to stop them.
It's easy to Monday morning quarterback...
Just because it's justifiable to use a technique to bypass a bedroom scene - which is likely a fairly common use and justification - that doesn't mean any use of that technique is justifiable for any reason whatsoever.
Huh? Monday morning quarterbacking?
@Lanefan said that a player could say they're not finished with a scene. I pointed out that a GM has the authority to declare the scene over regardless. That's not Monday morning quarterbacking.
It's justifiable in all sorts of cases, like the shopping scene. If a player is spotlight hogging, such as haggling with merchants for hours, I would say it is good GMing to say something like, "Buy whatever you like. Based on your Persuasion roll he gives you a 10% discount. Moving on..."
Similarly, if the players are attending a fair and the BBEG shows up and destroys it, it's too late to ride the tea cups no matter how much the player wanted to. Unless the GM is willing to allow a retcon, of course.
In any case, this isn't about what is and isn't justifiable. We could undoubtedly both come up with about a million examples illustrating both good and bad GMing using this technique.
My point was that it is well within the GM's authority to do so in a traditional RPG like D&D.