While I disagree with any sort of binary "we should do X and not Y" arguments, it is worth considering that once mankind has off-planet opportunities this planet will become significantly more dangerous.

One of the benefits of nuclear weapons is that they make everyone have a significant risk, including the belligerents who traditionally got to hide away, protected and immune. If the people who push the button, so to speak, have options beyond Earth, it makes the prospects much more dim.