Wo Fat
Purple Belt
If you try to down block a jab, and you're not a very tall person, I think you're going to eat some fist. So that's different than a lunge punch.
On the other hand, if you slip a lunge punch, the same technique slips a jab. Huh, wonder how that worked?
My point is that a down block has lots of valid applications, but if your opponent doesn't throw anything that would respond well to a down block, it would not be smart to try to use it. However, there are an awful lot of ways to respond to a lunge punch; some of those ways will also work on a jab. A well-trained martial artist will (hopefully) know when to use which defense.
One of the drills we've done in my dojo is pretty enlightening to me; you might enjoy seeing it. Our instructor will tell us to use a particular defense and then have our partner start throwing whatever at us. We have to make the defense he gave us work. Most of us can't (I sure can't) do it all of the time. But can it be done? You bet. Our instructor will demonstrate by having someone throw whatever punch and he'll defend it using the same technique. When we do this, we throw just as hard and fast as we can - he gets mad if we throw slow, off-target, or wimpy punches at him.
I understand what you're saying about schools that don't teach a full range of skills to their students. That's a shame, those students are being robbed.
But that doesn't really say anything about the applicability of TMA to the modern world, since we both agree that sub-standard schools aren't representative of TMA, right?
I teach a very similar drill. My students are required to perform a curriculum technique against anything other than a step-in lunch punch. Without fail, the "karate" stances are gone and a boxer's stance becomes the preferred "ready" position.
You mentioned that a slip works as well against jab as it does a kata-style lunch punch. I agree.
Always love seeing the adaptation of boxing into the karate.