Can't move pivot on planes.

Vertex snapping doesn't seem to work well for certain geometry setups. This is most noticeable in planes or other flat geometry.

1. Add a Plane to a scene.
2. Select the Plane.
3. Attempt to move the Pivot (hold V+MMB while moving the mouse).

Expected: The pivot should easily move between the 4 corners / vertices of the plane.
Actual: The pivot will get "stuck" on a corner / vertex.