The extend command will only automatically extend a curve to the first intersection with the boundary object; subsequent extensions are done dynamically.  See below.

Ideally in the example below, each additional click would extend the line to the next intersection with the spiral.  I am an architect, and this is based on how I would like to use the extend command and how it works in AutoCAD.  (I should add that I have all but totally abandoned AutoCAD and have been doing all drafting and modeling in Rhino).