The problem with "built in errors" in a sample library is that each time the note with the recorded error is used, that error will be present in an identical manner each time. In performing, that simply is not how errors occur (one can get into strings "wolf tones" but players learn to massage and work around any particular wolf tone they may have). The degree of an error will also change from performance to performance or phrase to phrase. Simply stated, error is never constant.
For example, I am thinking of a recorded error in ye olde EWQLSO Gold... a particular buzzy key click on a particular oboe note. Did that make the instrument sound more real? No, rather, it made the instrument sound more fake, or perhaps being played by an inept player (or an instrument that needed serious repairs).
As for overall library sound, and the comments that VSL needs to do new libraries... maybe so. But, the key question is what sort of sound is one looking for? All libraries have strengths and weaknesses. For me, I wish I had migrated to VSL much sooner... it is one of the few series of libraries that is not more or less strictly "out of the box Hollywood" and fits my interests much better than all the various cinema libraries that are out there. While it can do "Hollywood" it can also be used to create pieces in all kinds of traditional classical styles (Beat stated it far better than I can). Conversely, if I was first and foremost after the stereotyped lush "Hollywood" sound, there are other library options that would be well worth considering.