There are plenty of people with "diminished ambition" that don't do drugs. People with diminished ambition will do drugs "because" of their depression or whatever. The diminshed ambition does not come from the drug. I know too many people who smoked their way through difficult degree's to beleive that propaganda anymore. You're just regurgitating talking points, not giving an opinion based on personal experience. The stress reduction benefits alone make pot worth it's weight in gold.
I personally don't smoke pot because it makes me not want to do shit for 1-2 days after I use it. It's a massive demotivator for me.
As for other people, everyone knows someone who lost a year in college or who lost several years in real life with a wake-and-bake lifestyle.
And how can you miss the evidence? There are studies everywhere. Here are four that show pot to have an adverse effect on developing brains:
Block, R.I., and Ghoneim, M.M. Effects of chronic marijuana use on human cognition. Psychopharmacology 100(1-2): 219-228, 1993.
Brook, J.S.; Rosen, Z.; Brook, D.W. The effect of early marijuana use on later anxiety and depressive symptoms. NYS Psychologist January:35-39, 2001.
Wilson, W.; Mathew, R.; Turkington, T.; Hawk, T.; Coleman, R.E.; and Provenzale, J. Brain morphological changes and early marijuana use: A magnetic resonance and positron emission tomography study. J Addict Dis 19(1):1-22, 2000.
Brook, J.S.; Balka, E.B.; and Whiteman, M. The risks for late adolescence of early adolescent marijuana use. Am J Public Health 89(10):1549-1554, 1999.
Here are two articles demonstrating pot's diminishment of short-term memory:
Heishman, S.J.; Arasteh, K; and Stitzer, M.L. Comparative effects of alcohol and marijuana on mood, memory, and performance. Pharmacol Biochem Behav 58(1):93-101, 1997.
Fletcher, J.M.; Page, J.B.; Francis, D.J.; Copeland, K.; Naus, M.J.; Davis, C.M.; Morris, R.; Krauskopf, D.; and Satz, P. Cognitive correlates of chronic cannabis use in Costa Rican men. Archives of General Psychiatry 53:1051-1057, 1996.
This stuff is everywhere. How could anyone possibly miss it?