One group who will undoubtedly benefit in the future from the many sets of orchestral parts now posted on IMSLP are symphony orchestras -- from community orchestras to top professional orchestras. The parts on IMSLP will not only save them money that is usually spent on rental or purchase of PD performance parts, they will also enable orchestras to expand their concert repertoires to include little-known works that have long disappeared from the musical scene.
All orchestras have their own libraries of parts. In addition to countless copies of Beethoven Symphony No. 5 and Mendelssohn's "Italian" Symphony, many of them also own parts for lesser-known PD works. Some of these sets of parts are not presently stored on IMSLP. Some of them are hand-copied parts for first-and-last performances. These include both published and unpublished works.
Most music librarians of major symphony orchestras in North America belong to MOLA ( http://www.mola-inc.org ). Probably there are similar organizations of orchestral librarians in European countries -- although I don't know the names.
I would like to propose that the powers-that-be at IMSLP contact MOLA and music librarians of various orchestras to enlist their cooperation in the Orchestral Parts Project.
Specifically, a census of orchestral parts for PD works not currently stored on IMSLP could be compiled, with data indicating exactly where these sets of parts are located. Then, orchestra librarians who wish to donate scans of these parts could upload them to IMSLP. It wouldn't surprise me if grant money from some foundation would be made available to sponsor such a project.
I think if a large number of orchestras elected to participate in such a parts census, all sorts of long-lost musical treasures would emerge -- in addition to sets of parts for standard PD works. As far as I know, there has never been a comprehensive survey of musical works stored in the libraries of symphony orchestras. In the case of orchestras that go back to the mid-19th Century, such as the NYPO and BSO, many valuable sets of rare materials are sure to emerge from their libraries.
IMSLP would benefit, and orchestras would benefit.
MS