The problem with this is that low level hooks have to be processed and returned to Windows as fast as possible. If the hook call does not return quickly enough, Windows will evict (remove) the hook, taking it out of the call chain without notifying the application (S+).

Opening it up to plug-ins could result in Windows evicting the hook(s); an event subscription model really is not ideal for this scenario. Plus, people writing poor plug-ins or simply having too many plug-ins subscribing to the events could lead to delays, again causing Windows to evict the hook(s). Not only that, poorly written plug-ins could impact the amount of interference of mouse/keyboard input to the point where the user can tell mouse/keyboard events have a slight delay as the plug-in(s) are processing each event (there are a LOT of hook events/messages).

Now, an asynchronous subscription model would be fine, but you would not be able to control the return value of the hook calls made by Windows - it would be notify only, unable to prevent the event from being passed along to the OS/applications.