As I have read from the document on string pulling,
the path is smoothed by trying to reach a node up the path if it is visible,
This works well on a pure 2D enviroment.
But using a trace wold not be enough, as the
bot might try to reach a node, he can see, but a crack in the floor
or a close edge might make him fall down.
So do you check on the floor, if there is some kind of drop?
Again, it a good sollution for 2D Pathfinding, but
might cause a lot of trouble in a 3D Arena.

Here another interesting page from valve:

http://developer.valvesoftware.com/wiki/Navigation_Meshes#Introduction