That is not the argument. I don't know what the argument is, but it can't be that - it doesn't make any sense. If we voluntarily destroy all our samples, and some other nation doesn't, then there will be that much less smallpox. This is a valuable goal in itself, even if it doesn't mean that the virus has been completely eradicated.
No one who wasn't literally insane would try to use smallpox as a weapon, the infection would inevitably spread back to the country which initiated it, and the idea that we would need samples of our own to retaliate is preposterous. For one thing, the entire premise of this scenario is that this other country has just given us all the samples that we could possibly want. For another, we still have tons and tons of missiles and bombs just sitting there, looking for a way to justify all of the money that we paid for them.