I should start out by saying that emotional addiction can be just as harmful, but marijuana affects the chemicals in the brain and therefore brings about physical reponses that make it different from external influences (such as TV, or the DIS boards

).
Will weekend users lose enought brain cells to make any difference? Maybe not. I've seen diminshed capacity in too many people becuase of an over dependence on pot to be able to make a blanket statement that this drug is as harmless as drinking tea.
From
http://www.scvmed.org/scc/assets/docs/98416choices_0302.pdf
Every year, over 120,000 people seek treatment for
primary marijuana addiction. Yet even health care
professionals still ask whether marijuana is addictive. The
American Society of Addiction Medicine states that A drug
is addicting if it causes compulsive, often uncontrollable drug
craving, seeking, and use, even in the face of negative health
and social consequences. Marijuana meets this criterion.
Recent research shows that marijuana causes physical
dependence, tolerance, and a characteristic withdrawal
syndrome when heavy users abruptly stop using. In other
research, subjects reported irritability, stomach pain,
aggression, and anxiety when they stopped getting THC
(marijuanas main psychoactive component) orally. The
National Institute on Drug Abuse (NIDA) has found that
regular marijuana smokers suffer withdrawal symptoms
when they stop using, and may continue using to prevent
the irritability and discomfort associated with withdrawal.
According to NIDA Senior Investigator Billy Martin, The
fact that people do seek treatment for marijuana dependence
is evidence of marijuana withdrawal in humans.
http://www.scvmed.org/scc/assets/docs/98416choices_0302.pdf
Does the government go too far in banning the drug? Absolutely. But swinging too far to the other extreme will have serious consequences, too.