There are some valid use cases for special edges in a 3D model (e.g. emphasis using dashes and other patterns, custom width, etc.), but the majority of requests for this kind of feature come from people who are trying to create traditional 2D drawings using SketchUp alone, that is, not using LayOut. As @eneroth3 points out, the whole idea of “double line” is 2D because the second line must be offset from the first in some plane. Unlike some other apps, SketchUp does not have a concept of “working plane” so it has no native way to know how to orient the doubled line. Since SketchUp is not and does not aim to be a 2D drawing app, I agree that this could be an extension but doesn’t belong in the main app.