You are under-estimating the long-term traumatic psychological effects a bad trip can have. Not to mention the inability to properly control dosage in a large-area dispersion scenario.
The vector part is really interesting tho. How can you reliably dose a mass population in an urban environment? The limitations of gas have already been discussed (https://news.ycombinator.com/item?id=30685608). My idea about "cluster darts from drones" sounds 'good' but seems like it could have some serious technical issues / pitfalls. I think a "natural agent" like a virus is the best way.