That's always the curious thing about the Pirates line, and the PotC line that displaced it (and hopefully dies soon so it can come back). The last Pirates line was fairly underwhelming... right up until their last set, the Imperial Flagship, which is probably my all-time favorite set. It's absolutely beautiful, and could stand alone even without a single minifig. I had amazingly high hopes for the PotC line based on how the Pirates sets had handled things like the forts, the Flagship, and even Brickbeard's Revenge.
You're right about PotC in general. The Black Pearl was talked about all the time, but never really shown outside a few deck shots and the captain's cabin. The QAR is exactly the opposite, the look of the ship was featured prominently in the movie, and the set was basically a glorified canoe that bore a superficial resemblance to the ship. In my mind, only two sets in the entire line managed to capture even the most basic elements of a good LEGO set: Cannibal Escape and Whitecap Bay. Cannibal Escape because it was a decent little set for the money with figures that could be moved around, and Whitecap Bay for being a decent playset that exceeded its minifigs.
I think the big difference between LotR and PotC is that the major events are tied to some sort of location; I have seen the movies a few times, but cannot make it through Tolkein's books (I am a very seasoned reader, and know when someone is just playing with his language and not trying to tell me an actual story... that and you can only read so many dwarf drinking songs before you succumb to the urge and just get too plastered to read). Things that we remember are with a place... the fellowship was in the elven city, The Battle for Helms Deep, the watch tower, etc. Yes, they won't be as good without the figs, but they should be able to stand without them.
I'll use my favorite Toy Story and Indy sets as an example. For Toy Story, it's the Pizza Planet truck. Without the figures, it's still a great little truck. Add the figures and suddenly you're taken to the place in the movie. That's what a good LEGO set should do, especially when licensed. It should evoke some emotion and feeling, and even absent some figure, be a good thing.
For Indy, it's Shanghai Escape. The two vehicles are quite well done, but once you add the figures... well, if you can look at the car with Indy and Short Round and not say "Not time for love, Doctor Jones!" you are dead to me.
You know, if I see links to Brickshelf, more often than not, the first thing I click is the "Back" button.