I don't think the motivation issue will ever have an absolute answer. The variety of experiences shared in previous comments are a good example. One of the main reasons for this relativity is intent. The pot smoker who is just chilling down from a hard day of work is very different to the kid who just wants to briefly escape from problems at home. Each circumstance will also have an effect in neurochemical balance.

And then there's individual brain chemistry. Most people who use cocaine become super wired up. But I know a handful of individuals who snort it and then go to sleep.
I also know hardcore surfers who are "potheads" yet functional.
These are fit and athletic. While pot might mess dopamine levels, it is also true that the high level of physical activity involved in their sport helps produce plenty of it.

My humble conclusion is that establishing an absolute, one size fits all rule is dangerous .