I certainly cannot hear the music in my head
I have a 'reverse' problem: I hear lots of things in my head but can't get t out fast enough (making me loose most of it). And to my dissapointment, I can't exactly pinpoint what notes I'm hearing, so when I do remember phrases, pieces or whatever, I waiste time (and consequently loose idea's) trying to figure out what notes it all were.
I'm the same. I pretty much hear full scores playing in my head, complete with lots of different nuances and instruments flying about in the background. I can't notate/write properly - although I try and notate everything when it's finished in Sibelius afterwards just for the practice - and I'm not all that clued up on orchestration (though I've started hitting the books).
But VSL allows me to sit down with the extensive library and determine whether it was an Oboe or clarinet I was hearing, whether it was a horn or trombone swell, what percussive instrument that was etc etc. And then when I do find the sound, I look up the instrument in my "Sibelius Instruments" program or encyclopedia just to make sure that the instrument CAN actually do what was in my head!
If it weren't for programs like VSL, I'd have to stick with rock songs and electronics. But even if I was capable of writing a score with nothing but pen and paper, VSL is the only way to (1) play it back and hear it to see if it does actually work or (2) show other people what it sounds like who can't play things in their head by reading scores, or (3) score low-budget productions that can't afford an orchestra.