Interesting Markus, but I my opinion, Williams does just about enough to take the music away from the proposed original inspirations, but It does seem clear that in some cases though he might have had existing music in mind prior to composition, be that through a temp or from his knowledge of the repertoire. I have always noticed a similarity in places between his last cue in ET and Hansons' Romantic Symphony too, but I find the Jaws/Dvorak comparison a little too tenuous in the link above.
I take Williams (Bills') point about plagiarism, but am not prepared to outright imply that Horner did it purposely or as a matter of course. In my pro work, I have often lost work because of a refusal to get close to a temp and do understand the pressure involved, especially when it is your living. Having said that, sometimes I did get the job and often that was because I incorporated the spirit and emotion of the temp in a different and yet similar guise. At Horners' level, I can certainly appreciate the pressure he must have been under to deliver with tight deadlines and coersive pressure from the powers in control. He was in trouble from the Britten estate I believe for his shameful rip of the Sanctus in Brittens' War Requiem' in 'Troy', which dropped my jaw to the floor upon first hearing, but then again (although this is no excuse really for such blatancy is it?) he only had a few weeks (2 I think) to do the whole score as the original commissioned (and recorded) score was dropped.
I still do not think Horner is the villian here, although admittedly Bills' examples are compelling - I prefer to share the crime out amongst media ethics (or rather lack thereof) and practice as well as the participants willing to partake in such.