I'm not sure I get your point... is it that you can't catch fish on Intruders without a Skagit line? If so, then I've been dreaming. My long bellies do just fine. I think it is the light tips that Ed uses that are the point of importance with his flies. Sure the lines are cool, the casts themselves are efficient and effortless, I even have a line that Marlo gave me and occasionally I use it, but the bottom line is not how you cast Intruders - it is how you fish them.
Oh, maybe the most important point is that fish crush them