That's all fine and good, but your original argument was that breaking the law is irresponsible and frankly, when the law is stupid or immoral (as it is in regards to weed), the opposite is true.
And besides, of course weed impairs you, (a little bit) but if it really impaired you all that much I doubt I'd be working as an architectural drafter, doing complex trigonometry and math, drafting complex drawing sets, while the entire time, almost every day, high on marijuana. In fact, anecdotally, I work slower, with less quality and have a much worse at ude when I'm not stoned.