I've been indulging going on 42 years now, and it seems that it hasn't affected my inherent intelligence or ability to reason. Way back when (70's) I was a member of MENSA, an international organization for people with high IQ's (98 percentile on the Stanford Binet Intelligence Test) My IQ was then measured as 172. I dropped out because I don't play well with others.
Back in 2004 I retested and actually showed a slight increase up to 176. I hadn't smoked for about 2-weeks prior to testing the 2nd time in 04. I guess it's safe to say that long term use hasn't negatively affected my intelligence or reasoning ability.
I have also wrote and published numerous technical scientific papers over the years while being stoned and they were more than accepted by my peers. Once back in 1983 I presented a technical paper on organotins and condenser tube sheet macro fouling at Magill University, Montreal in front of a crowd of about 300 of my peers while I was stoned.
I even took and answered questions.
Nobody had the slightest notion that I was stoned, but they did find me a bit odd due to my numerous tattoos.
Yes, my short term memory is affected when I'm stoned, and for some time shortly thereafter, maybe about 1 day or so, but it springs right back with no lasting consequences. Smoking would always slow my brain and thinking down to the point of being able to more consistently focus, no kidding.
So, at least for me it hasn't affected me in any negative way associated with my cognitive processes.
I cant speak for everyone, but that's my story.