I think it's very dangerous to make too much of comparisons between the types of stories that appear in fiction and the ones we can develop for our D&D campaigns. The differences are far bigger than just a difference in win/loss ratio, etc.
Look at the motivations of characters -- how many fictional characters are deeply driven by their need to improve their skills and loot the corpses of their kills? And how many RPG characters are motivated in real, complex ways by Love, Duty, Honor, etc? (You may have that written into your character's background, but is that _really_ what drives him? Odds are it's a lot more about loot, XP, and just plain "winning").
Fiction and gaming are closely related -- and a DM can find all kinds of inspiration and ideas in fiction, but at the end of the day he's spinning a campaign, not a novel, and the two are different enough that comparisons like this one are not all that fruitful.
Having said that, I know there are game systems that make much more of character backstory, motivation, and so on. Game systems designed specifically to recreate fiction (like Dresden files RPG (powered by Fate), or any of the Cortex games, and so on) get closer to the mark than D&D does -- and I have a lot less experience with those games than I do with D&D, but it appears to me that those motivational elements become tools for gamesmanship as much as they do true motivators.
So, anyway, back to the original topic -- those sorts of "defeats" are often important storytelling moments because they give the bad guys some time to walk around on center stage. Even if they're minor villains, like the Trolls, they defeat the main characters, strut around on center stage, reveal something like their plan or their motivation or some other bit of story -- then the protagonist has an opportunity to turn defeat into victory. But in gaming, time when the PCs are not on center stage is bad time, not interesting time for the players. It's time when they're sitting back and watching the DM create story for them, and not working together with the DM to build the story.
I don't know about your players, but that's usually when mine decide to go take a trip to the toilet.
-rg