The MMC API offers much of the behavior provided by the Shell extensions. However, MMC adds the ability to build tools (through the document persistence model). Therefore, the bulk of those APIs required changes. The biggest changes are needed because MMC extensions are per-tool, not per-machine or per-user, so MMC extension data is stored in each .MSC file. If you are currently developing to the shell extension interfaces, a migration to MMC is straightforward.