Today I was surprised to learn these two classes never had this property. The reason I needed this is the down arrow, which is the only mouse icon you get when you hover the mouse over a column header on a grid. The reason this hurts is that the users tend to think the column is sortable, when they see the cursor change shape over a header, and then complain about it not being so. Also, having the icon remain an arrow on unsortable (and turn into a downarrow on sortable) columns would be an easy visual clue for the user to know which columns are sortable and which are not.

I've added this to the wish list for 9.0, but maybe it's not too late for 8.