Multi-category schedules could be even more powerful if Autodesk kept the "<Multi-Category>" option when creating a schedule to be able to include all categories in Revit, but add to that power by being able to select multiple specific categories while creating the schedule so that the user doesn't have to use filters to filter out a lot of stuff they do not want in a multi-category schedule.
Example: My firm uses the curtain wall system family to show windows, while simultaneously using them for storefronts and curtain walls. It would be nice to be able to schedule them in an "opening schedule", particularly when used for windows, when we have a lot of small projects where those things could/should be grouped together. If the user could select Doors and Walls rather than the multi-category schedule option they would have less to filter out.
Again, keep the option for multi-category schedules for those that make good use of it, but help manage the small projects by making this a function of schedule creation.