I prepair each instrument in a way that it also could be heard as a soloist.
That is a good point. If you make even a secondary viola chordal accompaniment sound like what you hear live players doing it adds greatly to the naturalness. And this is on parts that live players themselves would be bored by. As a sample performer you cannot be as bored as they are in reality by less-than-spectacular parts.
ALso, I really agree on the compatibility between different but similar instruments within VSL. This is one of the remarkable, great aspects of the library in how you can totally change an orchestration simply by substituting the same articulation names for a different instrument. It is a wonderful way to experiment with orchestration by the way. For example, I recently discovered that the bass trumpet sounded better, in fact perfect, on the third horn part of a piece I was recording than an actual third horn part. I would never have thought this in a live situation or in a written score. It is pure experimentation which is allowed by this consistency across instuments that VSL has. It is even more consistent within instrument groups such as the strings.
Here, you can originally record an entire string orchestra performance, with the biggest sounds such as Appassionata and Orchestral, and then create a completely different articulation groupp with Chamber and Sol simply by exchanging articulation names, and almost no tweaking because of the methodical sampling/naming of articulations. You can then add that smalleer ensemble on top of your original for extremely complex effects especially considering you can do further humanizing within the MIDI tracks as well as VI.
All of this applies even more to Dimension, which functions as an even great amount individual control and complexity. So with the various string ensembles within VSL now the individual line complexity has become really close to live ensembles. In fact, beyond some live ensembles in certain cases.