Maybe if you only count intelligent enemies. Otherwise asteroids, gamma ray bursts and planetary dynamics are also dangerous. A second planet would let us recover from all of those. Except for maybe a gamma ray burst depending on how close the second planet was. Even if you limit the scope to ways we may kill ourselves: Nuclear war would likely be contained to a single planet. A second would at least improve our odds.

Disclaimer: I very much agree with the sentiment of the comment you responded to. I don't even plan on having kids or anything. I just imagine a dark and boring universe deprived of humanity drifting off towards it's icy death with nothing to replace us.

Humanity faces many dangers, one of the largest being itself That doesn't mean we shouldn't do what we can to reduce or eliminate other dangers (planet level extinction event).

Also, even though it doesn't eliminate our ability to wipe ourselves out, that it's a a step towards setting that capability back a few decades is also beneficial (it should be very easy to identify and destroy long-range nukes in space).