Low cost, minimal technology, little risk. The playing field becomes even and the distribution of advanced technology can be increased. Now, the stakes are higher and even the monies invested are higher just like the stock market whose wealth increases by the numbers of players involved. The highest return on investment is in defense and with fiat money there is no limit to investment.
States acquire nuclear weapons for deterrence purpose and possession of nuclear weapon itself makes state rational enough to behave in a rational way. Again, there are less possibilities of nuclear weapons acquired by terrorist, except the myth of 'dirty' bomb. But, in case of drones, it will enable state as well as non state actors to actually use them, it will make warfare a normal phenomena. Secondly, terrorist can easily acquire it and make its lethal use against common people. So, pragmatically it is dangerous than the myth of nuclear catastrophe.
All the no arguments before mine have focused on military drones, such as Predators and Reapers. However, nobody looks at civillian drones. A drone strike from a Radioshack Surveyor Drone will do as much damage as a shot from a Nerf gun. Many drones are the size of my hand and incapable of harming anyone. The majority of drones are no threat at all.
Unless drones are equipped with nuclear weapons, they will never be a greater threat than nuclear proliferation. Drones are remotely controlled and their purpose is to engage in surgical strikes. Nuclear weapons are not surgical; they are all about total and mass destruction. That said, drones still need to adhere to a tight set of ethical standards.
A nuclear weapon is far more destructive than a drone attack. A nuclear weapon not only kills thousands instantly, but it also destroys the land and buildings in the area making it completely inhabitable. The long-term repercussions of nuclear contamination are far more wide-spread than a drone attack as they can also impact water, animals and many ecosystems.
A drone in the wrong hands can kill a lot of innocent people, but it can't wipe out a giant chunk of land the same way a nuke in the wrong hands could. They're both dangerous things that can do a lot of wrong, but one is on a much larger scale than the other.
Drones are a horrible way to dehumanize others and wage war with lots of civilian casualties. But even though I do not support drones, they are not capable of the same widespread utter disaster that nuclear weapons represent. Nukes can damage the world for years and years in ways we don't even fully understand yet. They are an extreme danger.