I find that the Grid preferences setting of (inch, mil, mm, mic) is used not just for "Grid", but for for everything except dimension lines. This causes me to repeatedly switch the grid units between measurement systems many times in the development cycle as I select drills, widths and spacing.
My board house has drill sizes in mm, but (being a backward american :-) I'm more comfortable with a mil-based board layout grid and trace width. For Library part creation, I fall back (forward?) to the metric system because that's what the data sheets all use, but Schematic layout is best done in imperial...
Expand the Grid units selection mechanism along the lines already done for dimensions: allow independent settings for Grid, Dimension, Drill, Width, Size and Spacing...
My personal use case would have