Sorry, but I don't want the people driving our school buses, trucks, their own cars, or flying planes to be buzzed on either dope OR alcohol. I understand the libertarian arguments and the financial arguments regarding freedom, product purity, the ability to tax, reduction of crime, etc.
If you would support mandatory drug testing for all of the above, with irrevocable license forfeiture if the test is positive, then I would consider supporting the legalization of marijuana. Note that we already have testing and license suspension/forfeiture for dwi, so this would be consistent.
Do you want your child taken care of by a stoned child care worker? If not, then why would you support legalization?