I suspect that would be done to be a fan of the characters. Extending the content to get the full benefits of their move.
Although I've very rarely worried about this myself, the scenes tend towards longer side, and the characters are more likely to run out of hold long before then run out of things they want to know; so they actually tend to hoard and treasure the holds like they were chess pieces to be used at just the right times. Sudden and unexpected ends of a scene do eat hold now and then, but I let them "stop time" when they ask and I answer, so even then they can get some last second intel.
Basically, in my games, everyone is malleable to some extent, if you know the right places to push; and the NPCs have shiftable agendas. Information is key to victory, or well, key to the victory my players tend to chase. They love having the upper hand, but I constantly push to make it hard for them to keep it. Read a sitch is basically a gold mine, and it provides the characters way to prompt conversations with npcs, to get people to open up, to be tactical about knowledge, or emotional responses. It allows them to read the other npcs like a book, but only a little bit at a time.
---Edit---
It might be that while NPC's wants tend to be simple, their emotional states tend to be very dynamic. There are many ways to sate a want for an NPC as well as a PC, many ways to put food on the table, many ways to lift their pride, many ways to despair. They're almost never black and white utterly horrible people, so when the the conflicts come to a head, or start there, the characters almost never know all the moving parts before they got there. These circumstances provide huge piles of opportunity for them to learn enough to change the entire scene with just the right choices of words, the right offers, to the right people.
They are things that could be achieved by Hot and Hard too, mind you. However, there are parts of the conversation still under the water. Will this happen again? Why did this happen in the first place? Was someone putting on a facade through the scene? How do they feel about you stepping in and ending it like you did? Good use of Sharp allows a PC to choose a resolution to a conflict with much more precision, often times earning admiration, fear, reputation, or getting a chance to decide if someone really does deserve to get a bullet in the head--or if they are better off collected for another purpose.
Basically, what sharp does to my game is provide the characters a way to understand the ticks, quirks, and mannerisms of people they study with an awareness otherwise impossible. It definitely adds to the game. Let me provide you a quick example using hold over time (which is the default in my games, rarely do they want to spend it all at once).
Two characters are just off talking somewhere, the scene isn't charged but the players are starting to have questions about each other, under the surface they're feeling each other out. The first character decides they want to know something, but rather then get the calm surface thoughts on whatever they were talking about, they reach out and grab the other one, growling and putting on a "mock?" display of violence and turning the conversation to exactly what they want to know. Then they both decide to roll Read a Person, but neither interfere, because they're not really trying to hide their cards so much as see what the other is holding. "Do I trust them, Can I trust them? I want to trust them." They want to know what the other is feeling, not just what they're saying.
Now the characters both hit a 10+. The first asks 1 right away, because they steered the scene into this for this piece of information. They get to learn something, and their conversation snowballs all by itself. The conversation becomes tactical, how to get the other one thinking about the thing they want to read from them, or saying the thing they want to know if a truth or lie. How are you feeling right now? Will be different if they're peacefully gazing out into a sea of grass, then if they're being confronted over their sins. The more hold they have, the more they can go after in that conversation. Often times they get much more out of this, because it wasn't just the hold that mattered, sometimes the thing that was standing in the way was the confidence/prompt to just ask a hard question to begin with, and the hold provided them the security that they would learn something from that answer, even if it wasn't what they wanted to hear. Many times they would actually get the answer and believe it without spending the hold, but it got them out of the shell enough to ask for that answer.
Now this was a very specific example, there are hundreds of way to do these charged conversational battles without actually using force to shake up the other person. The key is to get them off balance somehow, or you'll probably end up with more prepared responses. These things happen quickly in person, I tend to be rather quick on my feet in these types of conversations, so it also might be that the hold allows them to be sure about what they're getting out of it. Plus, missing these are also pretty cool, since I almost always turn the move around and use it in return to try to get what the NPC wants too. (that also tends to reveal things to the observant player)
Trust is a big thing in my games. Also yes, I think hold over time adds a great deal to it.