The first is moral. More people have been killed in U.S. drone attacks than were ever incarcerated at Guantanamo Bay. Can we be certain there were no cases of mistaken identity or innocent deaths? Those detained at Guantanamo at least had a chance to establish their identities, to be reviewed by an oversight panel and, in most cases, to be released. Those who remain at Guantanamo have been vetted and will ultimately face some form of legal proceeding. Those killed in drone strikes, whoever they were, are gone. Period.
The second is consequences. U.S. reliance on drone strikes allows our opponents to cast our country as a distant, high-tech, amoral purveyor of death. It builds resentment, facilitates terrorist recruitment and alienates those we should seek to inspire. Drone strikes may decapitate terrorist organizations, but they do not solve our terrorist problem. In fact, drone use may prolong it. Even though there is no immediate retaliation, in the long run the contributions to radicalization through drone use may put more American lives at risk.
Third, our monopoly on drone warfare will not last.