That's a good question, MrMonkey. I think the answer lies in examining another similar drug, sanctioned by society, called alcohol? How many raging alcoholics do you know that are still functioning, productive members of society? For me, the answer is 'too many to count.' My attorney generally spends the remainder of the workday after lunch with an open beer on his desk and if I have an appt. with him after 3PM generally I can count on him being a six-pack deep, provided he's not expected in court. This has no bearing on my opinion of his professional capacity or ability to interact with the world as an adult, as he's always done a stellar job of accomplishing whatever task I set before him regardless of how many beers he's had that day.
I see marijuana and its habitual/daily use occupying much the same place in the world. Especially growing up here, in the marijuana capital of the world, it's simply a way of life that as some people will go through life with a dependency on alcohol, others will do the same with pot. That doesn't make them children or unfit to walk the streets, it just means that it's their way of coping with the iniquities and disappointments which are part of daily human existence. It has become such a tolerable part of the culture here that most people will go to job interviews while stoned so their prospective employer can gauge whether or not their level of coherence while stoned is compatible with the job itself.
I suppose like anything, it really has to be evaluated on a person-by-person basis. In short though, I don't find being high on pot to be mutually exclusive with a clear head and the ability to "make it" in the real world. It all depends on your baseline mental state and tolerance for the substance in question, really. The culture you live in also has a great deal to do with it. Perhaps in other parts of the country there is no tolerance for people who may not be operating at 100%, but out here the most common reaction to an obviously stoned individual is to vicariously enjoy their good mood and show some respect for the fact that they're having a good time.