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).
I've added this as a feature request. Thanks for the detail.