> Some of Zimmer's themes do have zero flexibility, though I'll contend the
> new main theme of At World's End can be quite flexible, varying from
> brassy swashbuckle to calling romanticism. He's proven adept enough at
> making sure that you're still entertained by the music.

Using a theme in two different setting isn't "quite flexible", it's standard.

> Also, maybe anyone could write the music just out of the blue, but do you
> really think they could come up with music like that after watching a film
> that has no score and then having to make it up on their own. At World's
> End has a score that matches up quite well with what's going on the
> screen. Tell me one part that doesn't (I'm sure you're right, I'm just
> curious)

No, they wouldn't come up with music like that, thank god.
Alright, I'll tell you one part of the film that marks the difference between a serious film composer and Hans Zimmer.
Remember the scene in which Barbossa, Jack and Elizabeth meet Beckett, Davy Jones and Will on that small island to negotiate? A good film composer would watch this scene and recognise its importance. If I had to score not only this scene, but the film, I would realise that this is a key scene, and look for a way to make it stand out and resonate with the viewer. I would write a short, but poignant motif, maybe some kind of "fate" motif, that I could work with. That way I could play or hint at this theme earlier in the film, and musically lead up that moment on the island.
Maybe I could even make that motif part of another theme, a B phrase or something?

Hans Zimmer just uses this scene to incorporate a parodic Ennio Morricone hommage.

Such a film usually strcutures the music for you if you choose a leitmotivic approach like Zimmer does. What matter is how you put that on paper and where you put your emphasis.

But Zimmer doesn't emphasise anything. Davy Jones' theme is as bombastic as the love theme, Hoist The Colours or Beckett's Theme.
Zimmer can vary between exactly two settings: overboarding romance and dramatic adventure. In between is absolutely nothing but noise and the most obvious comedy scoring.