EMFs are a standard format for clipart used in PowerPoint. It is an excellent format for PowerPoint on Windows systems because the files are generally small and, when ungrouped, can be easily edited and customized. It is this ability to customize the graphic that was our reason for picking this format for a large portion of our frameworks.
EMF images are created using primitives from the Windows GDI+ drawing set, so they do not work very well on Macintosh systems, which convert these files to PICT using QuickDraw. We are currently working on providing our frameworks in additional formats, including formats friendly to Macintosh systems.