People convicted of drug offences have their future prospects massively hampered. People who choose to break the law must accept that they will be severely punished for doing so. Cannabis is often contaminated with inert or toxic components which cause more harm than the cannabis. A legal supply would be pure and therefore safer. Pure cannabis still has dangers.
Prohibition alienates a huge number of youths.
Most parents disapprove of all drug use legal or not. Legalising does not mean glamorising cannabis use should still be discouraged even if it were legal. Legalisation could make cannabis socially acceptable, and so encourage use. Countries which have experimented with decriminalisation have not encountered massive increases in use. Legalisation could encourage use of cannabis, which in turn could encourage the use of more dangerous drugs such as ecstasy.