You got my logic. If the cannabis is weak, one hit will not get you high. You are correct on that point.
Have you ever heard of someone who wanted one hit of weak pot? There are people who want only one hit of strong pot.
Ok, and the end result is what? Less consumption, which means less lung damage from inhaling smoke, and then what? This person is less of a risk to anyone compared to the same person that just consumed a massive amount of alcohol. MJ and alcohol behave differently in the brain. MJ makes you more risk averse (drive slower, think slower, don't take chances) while alcohol makes you risk-prone (give me my keys I'm not too drunk to drive!).