The problem with ADHD and friends cause is that my brain actually _doesn't_ fire on all cylinders. While I think I feel the same way as everyone else and I have the same cognitive ability, I don't have the energy to structure my work, complete it and keep track of things in ways other classmates/coworkers magically can.
Cannabis certainly has a calming effect that could lower the hyperactivity component, but it doesn't address the attention deficit part.
The reason the most common treatment are stimulants like Ritalin, is because it "wakes us up" and brings us ADHD people to an alertness level similar to what our peers have by default.