Legalization advocates will likely regard this as good news, but it may be a hollow victory:
Mexican prosecutors say the law will help the war on drug gangs by letting federal prosecutors focus their attention on traffickers rather than small-time users.
That is, the Mexican authorities plan to increase enforcement on the supply side. This will be disastrous, since the criminalization of drug trafficking is what generates violence and corruption. Worse, these evils increase as authorities intervene more by generating turf wars and battles with authorities that spill over to everyone else.
If Mexico proceeds as announced, therefore, the most visible evil of prohibition will increase. Prohibitionoists will then say, "We told you decrim was a bad idea; it would have been worse if we had legalized."
The right policy toward drugs is legalization of both the supply side and the demand side, both in Mexico and in the United States.