All film-composers are guilty of plagiarism in one shape or form. Just listen to Hans Zimmer's work. It is all the same drone. John Williams isn't original either. That does not mean that their music is not entertaining and have some spark of creativity (except for Zimmer, he borrows from himself way too much).

The main motif to Lord of the Ring's "concerning hobbits" was taken from the first measures of a Christian hymn. Sadly, I cannot remember the name of that particular hymn. Although I have heard it played many times.

As for Titanic, it is a great score. Honestly, it made the movie. I think it captured the human drama of the disaster. It sort of made the porn/love story believable. As it is, Star Wars would not be Star Wars if not for John Williams. Cutthroat Island would have really really sucked if John Debney hadn't composed that awesome score ( there is a bit of plagerism in that score). So Horner does not deserve harsh criticism. Plagerism smagerisim, I liked the score.