- Leverage in persuading scientists to become sources for foreign intelligence agencies, or to participate in sabotage.
- Delays development.
- Disincentivizes nuclear weapons research as career path.
The US and Russia experienced something on the order of 15 known nuclear weapons accidents despite attempts a command & control designed to prevent such occurrences. This doesn't include the unknown amount of weapons grade materials lost/stolen; or the possibility a few finished weapons may have been sold. How many rolls of the dice, by how many additional actors, before the world's luck runs out? From a risk management perspective, if there must be nuclear weapons (realistically) then not every country is capable of stewardship and possession of such weapons.
One need not take sides between, say, Iran and the US/Israel to make an ethical case for extreme efforts to prevent developing nations from obtaining nuclear weapons. The same ethical argument, however, strongly argues for open-sourcing nuclear stewardship technologies so that gaps in proliferation prevention are bridged.