IMSLP allows users to upload the source files (.sib, .mus, .ly, etc.) of their own typesets.
I think that it would be in the spirit of the IMSLP community to discourage its users from uploading scores in proprietary formats like Sibelius (*.sib, *.sco) or Finale (*.mus). These formats haven't been designed for score exchange. Morevoer, using such scores requires expensive legacy software available only for desktop systems. That means that more and more users switching to mobile solutions won't be able to read such files at all so the whole thing will be just a waste of storage.
Another danger of relying on proprietary software is that the vendor may either go out of business or drop support for your system rendering your whole work unusable. You could read this article if you're still unconvienced: http://blog.adamspiers.org/2013/02/25/m ... lius-saga/
Lilypond (*.ly) and Musescore (*.mscz) files look like better candidates because the corresponding software packages are open-source and therefore publicly available. Additionally, one can settle on the MusicXML format that has the strong advantage of being a very good format for score exchange.
Just my two cents...