[robertjwebb]

Procrastination happens when you reject a large long-term reward (the joy of finishing a project or solving a problem, or becoming successful in business) in favour of a smaller short-term one (partying, playing video games).

The latter activities are often more enjoyable while high so by introducing the option of smoking weed into the mix you are effectively increasing the appeal that the short-term rewards have, making it less likely overall that you will choose the long-term ones.

I suppose you can always use it as a reward ("if I work hard Monday -> Thursday evenings then I'll get high and chill out on Friday"). I'm sure that a lot of people who are successful do something like that (replace weed with any other vice or enjoyable activity).