You want your NPCs to be limited in their conversations, to only follow a set few paths, if you think of a way to play the game that's not been anticipated years ago, you don't want that to be possible
Yes - I want to maintain quality, even at the expense of flexibility. I would rather there were 1, 2 or 3 high quality paths through a RPG style game or quest than hundreds, thousands, or infinite ones that are 'meh'.
You can still have 3 high quality ones, it's just that maintaining quality at all the side chars is too expensive, so maybe the generated experience can reach the already "meh" level for those, thus freeing resources to be invested in the bigger paths?
That’s definitely a more interesting proposition to me (and a bit different to the parent comments).
It might well result in less repetitive background chatter & stock comments from side characters without taking focus away from the primary content. Especially if those background characters could react a bit better to progression in the plot.