Horner plagiarizes to an absurdly comical degree, but to say that he doesn't have his own personal voice is a bit too far. Whatever he re-interprets, you can usually tell it is him. If anything, I'm more bothered by how much he rehashes his material. He has his own voice, unfortunately that voice is usually a consequence of his rather limited musical vocabulary, as his infamous Danger Motif reminds us of in all the movies he's worked on.
Still, I don't see why you are comparing him to Herrmann of all people. They have completely different styles and work in completely different generations with different scoring requirements. Temp tracking was nowhere nearly as common for Herrmann's era as it was for Horner's. Herrmann is obviously more skilled, but I don't know if he would have been able to even survive in the same scoring climate as Horner, given how notoriously picky he was.
Maybe I'm just being kind to Horner because I've encounted far more shameless and unoriginal plagiarizers than him. Spot how many other's works have been ripped in this piece alone:
From Silvestri (Van Helsing) to Williams (Harry Potter) and then suddenly Herrmann (Cape Fear), and then ending with our dear lord and saviour Hans (PotC), of all people.