I'll want to have a look at that, but it'll have to be at home...no sound at work. :P
With any weapon, movement is the most important defence. But I get your meaning.
So, my Taiji training doesn't include that specific weapon (only the classic three), but my Xingyi training has included deerhorn knives (and I've done a lot with shorter weapons in my Arnis training).
[Me] *looks at deerhorns* Seriously, you want me to learn those?
[Teacher] It's something to do. <-- Really, this was his answer.
Anyway, they turned out to be more interesting than I thought and not as dangerous to myself as flexible stuff, so I'm okay with them (despite how they look, they're not Bat'leth). They're short, if anything shorter than butterfly knives, but I don't have a huge problem defending my lower body with them without even needing to drop into a very low stance.
How?
Without a weapon, just standing there, I can reach my hands down to mid-thigh. With the deerhorns, I can reach my knees. Put a little flex in the knees/hips/back and use my wrists correctly, I can completely cover the knees. With butterflies, it would be even more. So right there, the area that is out of my reach is essentially starts at my knees and goes lower.
Furthermore, when parrying a weapon, one wants to....
1. MOVE. Contrary to what movies show, one doesn't stand there and parry (unless in a press with sixteen other dudes, all of you carrying shields...I'm talking about one-on-one, here, and be it knife, sword, spear, whatever, this ALWAYS applies). If one doesn't move, one will have far more difficulty defending, and likely end up in a bad place for the guy's follow-up attack. So the targets of choice for the area of the body of which you are concerned are the legs, and they're going to be getting the heck out of the way.
2. Engage the weapon further out from the body. Picture a wedge extending out from your body. You parry close to your body, you need to push the other weapon quite a distance out of the way to have it avoid hitting you. Parry further out, and you can more easily deflect it so it'll miss. Which means engaging the weapon further. If it's aimed lower, you're actually catching the weapon before it dips as low as the tip will get (because HIS arms also start at the shoulder). Now, I don't mean 'try to catch it as far out as possible' because that reaaaaally opens oneself up to feints and adjustments in the enemy's attack. There's a happy medium.
On top of these two points (haha), reach changes when you attack low, simply because of the factor you note - our arms start high up. Point a sword directly out from your shoulder. It'll have X reach. Point it down toward someone's shin. It'll have the same reach along the length of your arm and your weapon, but because the angle has changed, the 'absolute' reach (ie, how far out from your body on a horizontal plane) has diminished. Before, maybe you could reach 6'. Now, because you're aiming down at an angle, you lose some of that reach - you can only hit the shin of someone standing, say, 5'4" from you (horizontally). As the reach diminishes, it means the person will need to move more to reach your shins. As he needs to move in more, it puts him in more danger; if his reach has dropped, it increases the odds of the other guy, say, stabbing him in the face while he goes for the shins.
And it's pretty easy - with a little experience - to lift the foot backward without moving much when someone goes for the shin (I do this automatically now, and yes, one also wants to try to parry because hey, he/she is not going to stab and just leave the sword there like it's a Kodak moment, that sword - if it misses - is going to still be moving and hunting for a target).
Now, if the other guy has a spear...ugh. Stab you in the shins all-day, every day, without putting their face in easy range. But you don't want to be fighting 'spear against shorter weapon' anyway, it's a crappy situation.
Okay, so the above? Does not mean I'm happy with Wing Chun and butterfly knives in general, or saying the above is a universal panacea to short weapon issues. Short weapons automatically come with a negative, in essence. Reach is critical. One can talk about 'dual weapons' and 'special parrying tools' until the cows come home, but a shorter weapon means the other guy can dictate engagement ranges much more easily, and force you to come through his range to reach yours. And range is critical, especially in weapon use (where it doesn't take a lot of force -- sans armor -- to wreck someone). And the business of 'once you get past the point!' only works if the other guy doesn't know how to use his feet (ie, MOVE, and if he doesn't know how to use his feet this is the exact kind of enemy you want - one with little to know experience and natural ability).
My personal feeling is that short weapons were secondary weapons or stuff that was simply more easy to carry/hide. They were emergency things that were 'better than nothing'. Proof is in the pudding; how many actual battlefield weapons were short or 'twinned', other than 'shortish sword and shield'? ^_^
The shorter the weapon, the more one MUST depend on movement to save oneself, though movement is always a critical point.
As for Wing Chun, the Wing Chun schools *I've* personally visited and examined seem to have the idea that everyone else is going to be using Wing Chun (this 'practice against your own school only' thing is not limited to WC schools....) and as such their stuff has become more and more locked in place over the years. I've actually had WC practitioners patronizingly explain to me that if someone brought them to the ground, they would simply use WC principles and striking to defeat that person on the ground, despite an utter lack of any kind of groundfighting training.
:$
I'm sure there are good WC schools somewhere that teach resistance training against stuff that isn't done WC style, and who can and will accept that in a real fight they shouldn't 'stick to stance', so this isn't a blanket comment on WC. ^_^
But if in your experience (ie, where this question is coming from?) they're standing stock-still in stance and expecting to parry everything thrown at them with their dual butterfly knives, they're in for a rude awakening against any opponent who actually has some experience with weapons and
moves their feet. If they're not at least somewhat flexible in their movement, similarly bad things will happen. I feel it would be more about 'lack of movement and flexibility' than 'reach of weapons', but.
And yes, someone with a much longer weapon might just be able to poke them in the shins. ^_^
...did this answer the question, or did I just ramble on like I sometimes do? ^_^