First, such negotiations will legitimate one of the most dangerous regimes on the planet.
And our policy of shunning our "enemies" has been wildly successful? How has this worked out in Iran and Cuba, to give two examples?
Second, embracing Ahmadinejad and his mullahs in this way can only alienate our natural allies: the people of Iran.
Actually, the most likely way to alienate Iranians who are sympathetic to the West is to bomb them.
Third, the adoption of the negotiating track effectively forecloses other options for dealing with the danger posed by the Iranian regime.
Frank, if anything precludes other options, it's the decision to go to war. That's why it's considered by most people to be the last option on the table. Clearly it's not the last option for neoconservatives, it's the first. That a conservative would think that a willingness to negotiate rather than bomb represents a "costly diplomatic error" shows how little our current leaders know about foreign policy and the effectiveness of diplomacy.
Thankfully, not all Cornerites subscribe to this madness. Stuttaford has seem more and more disillusioned as of late.