...I admit the combination of skills required is very demanding: how to use a sequencer and a sampler, knowledge about sample libraries and special tools for them, knowledge on how to translate a score into a sequencer part. Also, it's almost mandatory to have a target recording of the 'to-mockup' piece, mixing/mastering skills and good ears. Ah, also absolute determination to finish a huge work and near-to-infinite patience.
I believe there's a lot of misunderstanding around as to what this process actually involves. To achieve what we're talking about, it is NOT 'knowlege on how to translate a score into a sequencer part'..., but quite simply, musicianship.
The timing of every note, the shaping of every phrase, the interaction of all the parts, the rhythmic pulse, musical intensity, direction, flow... These are musical elements. The knowlege skills you mention are the equivalent to the instrumental skills needed to actually play an instrument. But we mustn't confuse practical skills with musicality. OK, so someone could be a fantastic musician and not feel able (or not want) to express their musicality via samples on a computer. Someone could know everything there is to know about sequencing and sampling, but if they're not a musician, the result will be unmusical. Equally, there are highly polished instrumentalists around who don't play musically! A musician - first and foremost, that is - with the interest and determination - can do this and achieve musical results.
Only one thing scares me: sampled mock-ups done UNMUSICALLY! These are dangerous in my opinion, because the audience is getting more and more used to hearing unmusical sounds passed off as music. THAT could undermine the true art of musical performance in my opinion.
Simon