The risks of nuclear weapons aren't theoretical. Nuclear weapons have killed people, destroyed infrastructure, and contaminated the environment. The Limited Test Ban Treaty was put in place after radioactive fallout from repeated nuclear weapons tests made people sick.

So in fact we've done exactly what you suggest there's "no point" doing - used the things and then had a discussion after the fact about limiting future use of them.

FWIW, before any nuclear weapons had ever been tested, a risk was identified that the first one might trigger a self-sustaining reaction in atmospheric nitrogen and destroy the entire planet.

Faced with such a scenario, is the prudent next move:

a) blow one up and see what happens, or

b) do whatever you can to be sure it won't happen before conducting the first test, and make sure the confidence in the calculation is very very high

because I vote for b, and so did Teller.