Are the use of virtual instruments in film music composition having a positive negative or both on the overall quality of film music?
Pros: Have access to instrumentation you otherwise wouldn't, or couldn't play if you did.
Cons: Reduces the human feel by reducing human error. Some composers feel like using the same sounds as someone else is a bad thing.
I think you're asking something as general as "Is using a screwdriver to build this a bad thing?"... VSTis are a tool, nothing else -- you either do well with them or you don't. There are techniques to correct common "problems," and there is almost always something to use.
In any case, this is a sound design forum, not a composition forum, so we're all a bit biased when it comes to using something canned or recording our own stuff.