The first part is a defensible position, but the second is wrong. First of all, because the previous two pacts that drug gangs have tried to hammer out (in June of 2008 and January of 2009, I believe), with the government's support, have not held, and the country has gotten a lot more violent. The biggest reason for that is, in my estimation, the fact that the trafficking industry is way too fragmented for any group to call a nationwide truce that means anything. As much as you hear about the five big cartels in the media, it's easy to draw a conclusion that if Calderón promises to let the five leaders go about their business, and the same five leaders agreed to not kill each other, then Mexico would turn into Costa Rica. But there's is nothing in the last five years to suggest that such a scenario is possible. Mexico's drug trafficking industry is not a battle between a few armed groups, but a wide open free market, a cauldron of splinter groups and new players and regional heavyweights. That's why the most "dangerous" gang today is one no one had heard of five years ago, and the most "dangerous" gang five years ago was one that no one had heard of ten years ago. This is part of the reason that I think the benefit of not using the word cartel goes beyond mere semantics: we simplify the situation far too much by classifying hundreds of different drug gangs into five cartels.
But even supposing that the tacit pact was a viable option, it would still be wrong. First, there's the moral issue; a government that establishes a pact with criminals, tacit though it may be, loses its moral authority to act on other issues, like strengthening the corporate tax code. If Slim armed a bunch of henchmen to start dropping bodies in response to a telecom tax in order to come to a tacit agreement, how would that be any different from the drug gangs?
Furthermore, the tacit agreement that persisted in the 1980s and 1990s was a) a lot more violent than is remembered today, and b) led to the disaster of a mole for drug traffickers working as Vicente Fox's travel secretary, among other achievements. This is something that was covered in De Las Maras a Las Zetas and last week's column from Macario Schettino: a lot of people in Mexican government, especially in the Fox administration, were slow to realize the threat that drug traffickers posed to the nation's democracy, not specifically because of the violence, but because of its corrupting power. That's not to say that Mexico is within light years of being a failed state, but leaving drug traffickers to their devices under a tacit pact, while it might lead to less violence, increases the threat to Mexico's government.
Update, 10-3-10: I agree with this much less now than I did when I wrote it, basically because I think misunderstood how "tacit" the pact is to be. That became clear upon reading a fuller explanation in Castañeda's book. My mistake.