This is what I don't understand about Wing Chun lineages that have a one leg forward type of stance as their fighting stance and say that the YJKYM is just a training stance. In our lineage the YJKYM is the fighting stance , we spend all our time in it whether we are doing SLT , chi sau or whatever.
By contrast, in the "WT" tradition which is my foundation, you most often find yourself in the "leg forward" position when fighting. I believe this is a natural position with parallels in boxing and escrima footwork (think "shuffle-step") that allow you to pursue and close on your opponent when necessary. Otherwise you can find yourself either a stationary target or engage in a sort of "waddle step" which I don't find very effective except for very short range adjustments.
Mook, I also used to be one of those you mentioned above who described YGKYM as"a training stance". I no longer do this, since I've come to see YGKYM as the fundamental "core" stance and structure from which all our stances and steps emerge and return. It seamlessly morphs into a "sideling" or turned-stance, a leg-forward front stance a side-stepping stance, a step to the rear and pivot, or any other WT stance, and then goes back to YGKYM in a single effortless step. Moreover, each of these stances and steps contains the same adduction, root and "feel" of YGKYM. So, instead of the "training stance" you might more accurately call YGKYM the "mother stance" in our system.
Finally, regarding your observation that the stance you train in most should be the stance you fight in, I agree. And furthermore, I'd point out that just as you most often see our lineage's practitioners moving in and out of the advancing-step or "dap bo" (or what you called the "leg-forward" position) when sparring, this is exactly how we train, both in drills (lat sao) and in our chi-sao. So our training footwork does match our fighting footwork. And once you close and engage YGKYM is most often adopted. It is not a mere "training stance" at all!