I should also probably add : merely being able to convert a line into a guide could be very useful. This might sound redundant/unneeded, but I find the input for creating lines (click first point, then click second point) to be a bit less error-prone than the click-drag input for creating guides (using ConstructionLine).

Or ... perhaps there is a way to create Guides in two clicks rather than click-drag ? This alone would probably solve my issues with permanent guides, and that would be even faster than having to convert things.