The european approach (de facto decriminalization) doesn't solve most of the problems americans are concerned with. Basically, the last link or two (user and/or retail supplier) are decriminalized, but the rest stay illegal.
This keeps production and supply in the hands of actual criminals that deal across countries. The legalized links are operated by borderline criminals and shady people.
It makes life better for the consumer (I have no problems with this), solves a lot of the policing issues but doesn't tackle solve organised crime, quality issues etc. If you are only interested in legalising marijuana, this policy doesn't create the wall between soft & hard drugs either. That is, marijuana stays in the "drugs" category and doesn't enter the alcohol category (where regular users will be appalled at the thought of their adult children using, for example, marijuana).