All the threads referenced are 2 to 3 years old. A lot has changed in the MusicXML world in the meantime:
1) The format is now supported by over 150 programs, including nearly every notation editor in widespread use, whether proprietary or open source. Many readers are available on Windows, Mac, LInux, and iOS. See http://www.recordare.com/musicxml/community/software
for a full list of software.
2) The compressed .mxl format introduced in MusicXML 2.0 takes care of the file size problem of the original uncompressed format. It also makes it easier to open MusicXML files by having a unique file extension that distinguishes them from other types of XML files.
3) Formatting support has greatly improved in MusicXML 2.0 and 3.0, as well as in the supporting software from programs, so the quality of the import has improved. PDFs of course still have the most accurate formatting, but offer no interactive capabilities for playback, transposition, analysis, and the like.
Keeping PDF as mandatory makes a lot of sense, but I am not seeing a huge administrative cost to adding compressed MusicXML .mxl files as a second file type. It seems that offering an interactive format would indeed provide maximum utility to IMSLP users. Since these files need to be created from new typesets, they will of course be dwarfed by scans at first. However, that may well change after several years.
With the maturation of MusicXML technology, could IMSLP please reconsider the possibility of offering MusicXML files together with PDF files?