Maybe it makes you dumb when you smoke. But I have seen others that it makes them more creative. It increases dopamine and serotonin and other neurotransmitters. So for someone that is ADD it can help increase these neuro chemicals opening up other areas of the brain.
Helping with focus, attention and creativity. Yeah and I know people who have worked for security agencies that are savants in the area of tech. So your statement is inaccurate. In fact many agencies hire hackers that have been caught. Most of them were major drug users. Not just pot but meth and coke.