just another day there was a thread related to sostenuto pedal as well, and i keep wondering why so few pianos do have it, since there are some languages that result impossible to render in its absence.
the foremost example that comes to my mind is debussy. a lot of his works requires that pedal, and there are no alternatives. no pedal, and you lost polyphony; right pedal, and all turns into a mess. even with half or 1/4 right pedal, you lose the precise articulation that he asks for.
the use of the sostenuto pedal goes even further. i'm exploring it's usage on some bach works with interesting results, and that's only one example.