walkplanes are used merely for positioning the 3D characters in the world and for the 2D<->3D conversions. For navigation one has to use the blocked areas and waypoints.
So, if our actor stands inside the walkplane and we click outside, he is walking only to the border of walkplane. Why? Because the border of walkplane *should* terminate walking? I think so. This is what we expect.
But if we have for example U-shaped walkplane and want the actor to walk from one arm to another, he walks across the borders and it is NOT what we expect.
As we can see: in some cases walking outside the walkplane is prohibited whreas in some cases it is allowed. 2d blocked areas used together with the 3d geometry makes no difference.
Anyway, defining true walk areas in 3d world could be much easier by simple "walkmesh" then by tenths of overlaped blocking boxes.