I would have to agree with Alasdair's concerns. I raise the issue even above the core MIDI engine (even though I share these concerns and have expressed them in the past).
I often wonder how long my awe with RealTracks will last. They are impressive, but not that impressive; they are good, but not that good...
There are many times that I have to thow out a RealTrack instrumentalist because he cannot follow the leadsheet in a reasonable way--either that or dumb down the chords significantly; e.g., the Pedal Steel, which is good for only the simplest chord patterns. In other words, the RealTracks are driving the arrangement and not the other way around.
The RealDrums substitute great sound, at least for a while, until you notice that a lot of creativity went into the MIDI drum pattern underneath that actually follows the style, not some standardized conglomeration. I find myself forcing MIDI drums more often than not.
Another example of low-hanging fruit left to rot would be the MIDI soloists. Virtually unusable for anything that I know of in their current state, nevertheless these soloists had a lot of potential. We will most likely never know how MIDI soloists could have been developed algorithmically to play like the RealTracks without needing the RealTracks in the first place. I would have been happy to find a Pedal Steel sound that would work with a soloist that modeled the playing of a Pedal Steel player precisely. The elusive fiddle player, no doubt would be hard to find but certainly worth the search; we'll probably never know the MIDI contribution in counterpart.