For one it is possibly an illegal substance, second, it affects your brain in ways that would require a lot of research before you commit to it, after all, a programmers most critical resource is their brain.
Now, I know some people that function very well in spite of using significant amounts of pot, but I also know a much larger number of people that managed to turn a mediocre career in to a non-existant one because of their use of pot.
If you want to advocate the use of drugs I really don't think HN is the place to do it, if only because it is not exactly legal in most places, but most of all because you are possibly sending someone from the frying pan in to the fire and you won't be around to pick up the pieces should it go wrong.