You're right, Herrmann's music did become the basis for many temp tracks- but in his later works with the Hollywood Brats, he was only able to write the music that he did because he was so revered by them. Scorsessee wanted something like Ghost and Mrs. Muir or Psycho for Taxi Driver, and was given something completely different. And DePalma, who dared to temp track Obsession with Herrmann's music before presenting it to him, pissed off Herrmann so much that he demanded the temp music be removed after just being shown the opening titles! And those are just the times that Herrmann successfully worked on a project, let us not forget the fights he got into with others from that generation like Kubrick on Lolita or Friedkin on the Exorcist. Herrmann was very skilled, but also very lucky in his last years. Horner never had the same luxury. Reading about his work experience with Cameron on Aliens sounds like the type of thing that would drive most composers suicidal. Of course, plenty of others had done better than Horner despite being put in even worse situations at the time (most notably Goldsmith), but I still don't think it's fair to compare him to Herrmann.
Still, Horner is something of an arrogant scumbag who has said and done some below-the-belt things that I deem unforgivable, like that time he replaced Gabriel Yared on Troy and pooped out his usual terrible rehash album for the movie, and then publicly badmouthed Yared's music for being "too old-fashioned". It's sort of terrible for me to say this, but his plane crash was pure karma at work.