I don't know if I've got ADHD or the Tism, but I find I can concentrate and work longer with a joint on the go. That said I work in art and both my parents aren't right about the head so the chances of me having a normally functioning brain is slim.
Generally speaking though without weed I'm far, far more prone to picking my work to pieces as I go, getting frustrated, restarting the project numerous times and clocking bitterly out after three or four hours. Meanwhile stoned me can keep working for 20 hours stopping only to piss. Don't get me wrong I'm still constantly scrapping and reworking, but I'm chill about it and accept it as part of the process, it feels more like meditation than banging my head against a brick wall until something gives.
I could work without it and I do for all small projects, but when I've got a demanding, HUGE design that's going to keep me occupied for a good 10-60 hours I'm going to need something that can root me to my pen like the goddamn human printer I need to be.
This works with art (probably writing and music for others too, I wouldn't know) BUT given any other job in life it's likely to be more of a liability as you're casting asides your sharpness in exchange for a dissociating mugginess.
Not many jobs require that weird zen state though and there are many people that make excuses for their personal poison for me to buy, even for a second, that it helps your average Joe.