Aside from the fact that in makes no sense whatsoever to down block using your arm against a leg (and that it is slow) why would anyone “block” period?
First it makes more sense to avoid and or redirect
Next it makes more sense to strike
Finally it makes more sense to meet the attack than block
I answer the question posed why would you ever teach it as a block to anyone. It makes no sense even to a beginner – they do it because you tell them to not because they ever believe they would actually use it, because after trying it just once they would see the problems trying to use this block. I know this was the case for myself and ever other karate-ka I have ever know. But we were told we were beginners and just didn’t understand or in some cases didn’t even have the right to question we were after all children in the martial arts regardless of age of intellect.
I teach at least 7 applications using the standard “down block” and not one is a block – strikes, releases, takedowns, locks but no blocks.
I teach at least 3 applications to beginners because when they see real value in a technique an application that works they will practise it and it allows them to visualise to prefect the technique during basics. As well it allows me to use basics and moves from forms as the basis of self defence, linking basics, kata and self defence creating a continuum that allows the student to see value in ever move they are taught and thus a reason to practice ever move repeatedly.
Anyway just my opinion feel free to block and or not teach the real meaning behind a technique but I think you are doing your students and yourself a disservice