There should be harsher punishments for people found using or being in possession of it. Just because alcohol and cigarettes are legal doesn't mean it would be a good idea to add another bad thing to the table.
Many drugs are used to aid in illness, and I'm down with legalization for that purpose.
Legalization for recreation will only increase usage percentage... and while I'm a firm believer in freedom of choice for each individual, if it has a negative impact on others then I tend to lean toward substance control. Rate Increase in DUI would surely climb, by how much no idea, but any increase % would be a bad outcome.
I think legalizing weed in France would actually reduce the amount of people consuming it since many people start rolling a one mainly because of the fact that it isn't legal and it is basically "underground" and "shady", even though your grandma might as well smoked 'em up many times ago.
Basically, make a substance legal reduces it's overall consommation, inform people better about what it is, and it gives it better regulation than just making it illegal, and that applies for weed as well.
TL:DR Making weed legal is better for France than to prohibit his usage.