A year ago, I would have agreed 100% with what you have said and how you have phrased your thoughts. I still agree in large part, but last year I had an experience that placed some doubts in my mind about how what you have said could be interpreted.
At a sword seminar last year Yang Zhen Duo corrected my back leg two or three times from what I had thought of as a "slightly" bent leg to what seemed to me like a "straight" leg.
In trying to make consistent all that I have been taught, I have since tried to break ingrained habits and straighten my leg fully. In doing so, I strive to deliberately extend and loosen the knee joint even in the "straight" position. by the way, I do not mean that I attempt to hyperextend it by bending it past a straight position so much as that I imaging pulling the joint apart and creating space in it. This prevents the joint from locking, but does not require me to consciously "reserve" any power in my knee or consciously stop my leg at any particular angle.
I do not know if what I am doing now is correct, but I have made two surprising discoveries in changing my form in this manner. I now realize that in my earlier practice, I was not fully opening up the kua of my back leg, since keeping a distinct bend in the knee and avoiding the challenge of extending the kua felt easier and more "comfortable." My flexibility, which is still abysmal, has increased greatly, at least in this respect.
My second discovery was that although "moderately" bending my knee avoided locking it, it had allowed me mentally to "lock" it in the bent position, since extending and loosening the joint in this position was not as straightforward. The feeling I now have is that I am allowing my leg to deliver maximum power without having to consciously restrain it. It is like the difference between trying to stretch a rubber band to its maximum and being restrained only by the amount of energy put in to the process, as opposed to trying consciously to stop the stretch at a particular point.
I had wondered whether to extend this change in the knee joint to the elbow, but someone who I occasionally study with pointed out to me that fully straightening the elbow tends to force the shoulders to rise up and lose extension. As a result, I strive to retain a distinct bend in my elbows, but more or less straighten my knee.
Of course, a lot of this analysis depends on what one means by "slight bend" or "straight leg," and I cannot read your mind or see your form to know what you mean by your posting. I have posted this not so much to disagree with anything you said, but to point out that a year ago I would have interpreted some of your words in ways that were apparently not appropriate in Yang Zhen Duo's eyes.
In looking at pictures of Yang Zhen Duo in his book and at Yang Jun in their video, however, I must admit that their back legs look more or less straight to me. "Straight, but not straight" is how one of my teachers has phrased this. What I take from this is that I should perhaps concentrate more on extending and loosening the knee joints and kua, rather than worrying about the angle of my knee.
As far as the vulnerabilities of straight legs to attacks from the side, etc., could it be that we may be putting too much attention to the end point of postures? Although I try to perform Yang Zhen Duo's form in as large a frame as my flexibility allows, my mental image of the applications is quite specific. As a result, I would alter their performance if there was any threat from the side, transformation of my offensive energy, etc.
Good post, and interesting thoughts. I am curious what others have to say.