Also, when you do an Air Drop (like St. Mere Eglise), if any part of a dropped unit is off the board it didn't make the landing. In the same way, if any part of a unit is in one of those half-hexes, it didn't make the landing. Unless you are playing overlord, those half-hexes are off.
And furthermore, if you are playing Air Rules, they are not viable entry points on the map. But because they are still there, the full hex immediately to the left or right of them are not considered viable start points either when bringing the planes on the board. In order to count, you must be a full hex physically on the edge of the board.