1. Schizophrenia - There needs to be more studies done, for now it's correlation not causation. Also marijuana with high levels of cannabinoids and low levels of THC show increased antipsychotic and neuroprotective properties. Not all plants are equal, some have high THC, others, like the strain Charlette's Web, are extremely low in THC and high in cannabinoids, which reduced the recurrence of seizures for Charlotte.
http://en.wikipedia.org/wiki/Charlotte's_Web_(cannabis)
2. Lowers IQ - The results of the study you're referring to are currently being challenged, the study did not prove causation, it showed correlation without regard to socioeconomic factors and involved only adolescent chronic users.
3. Lung cancer - I'd like to see one person who only smoked marijuana and developed lung cancer as a result. Aside from that, there are other ways to use marijuana, namely with a vaporizer which would not burn the plant, meaning zero carcinogens, or as edibles.
It's disingenuous to only list the possible negative effects and not list any correlated positive effects.