Cannabis allows connections to be made between concepts which normally seem unrelated.

Which leads to the problem of thinking up connections between ideas that are actually unrelated. Creative, interesting, neat-sounding insights are not always true insights.

Also, I suspect that different brains respond differently to this kind of experience. I know that a friend of my parents' is a mathematician by profession and frequently smokes marijuana to "help his thought process." As for myself and the small amount of personal experience I have, the feeling of being high isn't an unpleasant one and can even be fun as an experience in itself, but the quality of my thought processes drops quickly and if I'm actually trying to accomplish any kind of specific goal, being high (or drunk) quickly becomes irritating.

I do think your idea might be valid in the sense that anyone brainstorming under the influence of drugs is likely to produce more ideas than someone who is sober. Even if a lower percentage of those ideas are useful, someone whose sober thinking process is rigid or self-censored, and who thus has trouble coming up with any ideas during a brainstorm, might benefit from a little de-inhibition.