Two points in addition to Seth's list:
6. controlled quality (a legal trader is also responsible for his product = less risk of toxic substances like lead dust, or hard drugs)
7. the chance to have actually less young users
Germany and the Netherlands have a similar social structure. In Germany some states have decriminalized small amounts of pot. The cops
have to take it away, otherwise they would break the law themselves. If it goes to court the judge
can close the case under certain requirements (1st time drug offender, minor amount, no selling in schools, no smuggling, etc)
Contrary to many myths, marijuana is still illegal in the Netherlands. I've been told it would be very hard to legalize in regards to international law. Basically it's about some WHO agreements most states have signed where they state to do anything to fight against drugs, yada, yada, yada
So what did the smart and pragmatic Dutch do? Nearly nothing! No laws were abolished, no walls of text of new legislation was written. The police just has the simple instruction not to go against small possession. The Kofie Shops (
tolerated pot bars) are allowed to have a little more generous amount than a single individual.
Some 20 years ago I was on a public forum where the police chief of Amsterdam was present and answered on the question how they control the shops: "We don't control them. If a shop sells hard drugs or alcohol (!) we get tips from the scene. Then we raid and close it."
Many "control freaks" in the audience stood there with open eyes

, not believing how simple and non-interfering it can be and actually work.
(Maybe the police procedure changed today, but back then they said it worked well)
The result of two comparible countries with different marijuana laws: the percentage of adolescent users in the Netherlands is in fact smaller than in Germany. One should never underestimate the attraction of forbidden fruits, especially to juveniles. I don't have any statistics about adults, but on a limb I'd say the percentage is about the same.
Just a little food for thought for the "If pot's legal, all our kids will become stoned slackers"-crew.