From the support ticket I just submitted:
When Sorter is running and Spaces is enabled, the current application will remain active when switching to an empty space (e.g. using arrow key shortcuts) instead of making Finder active. This makes it necessary to explicitly select Finder (e.g. by clicking the desktop) instead of it being automatically activated by a space switch. Disabling Sorter restores the correct Spaces behavior.
People using Spaces might like to be aware of that.