If you want to argue for a backup planet, you should invest your resources into long term hibernation biology and then send the humans to an actual Earth II in another part of the galaxy. If you truly believe humans are at risk of extinction and need another planet, finding an actual acceptable planet to colonize is a better idea.
Interstellar travel is certainly a very very high risk concept all by itself. We don't even have the ability to send unmanned things to another star, nor do we have the ability to have humans hibernate for any period of time on earth, nor do we have a clear path to either of those technologies.
Moreover we don't know where any earth like planet is. Even if we nailed interstellar travel, chances are we'd arrive at another star system and discover that we didn't have the things we need to colonize the planet (do we need to live in domes? Or floating cities like on Venus? Deal with hot temperatures? Volcanoes? Is solar power available in high quantities or not? Assuming we are looking for somewhere with an atmosphere, how strong winds do we need to design for? What chemicals do we need to be equipped to filter? And so on and so forth).
Nor is extinction the only goal we are talking about here. We also benefit immensely from communication with more people. We can communicate with mars, but not people on other stars. We benefit immensely just from the inspiring achievement, which we don't really get if we (and our children) won't live to see the success or failure of the mission. And so on.
Successful establishment of a colony on mars is of course not without its own challenges. I think the risks I mentioned at the end of my previous post will mostly be about what it takes to establish a colony once we can get their, not the getting their. But the chance of success is obviously far higher with current/near future technology, and the value of success is greater.
Huge, absolutely huge. We are resource constrained on earth. We are at risk of extinction from a huge number of not-that-small probability events. We could all do with some inspiring things in our lives.
It's so huge it's hard to quantify, because it starts bringing in to question fundamental values. What do you think we, as a collection of smart and capable people, as a nation, as a civilization, as a species, or otherwise ought to be trying to achieve?
The maximum happiness for the maximum number of us possible? Gaining access to another planets resources and a defense against extinction events is almost certainly a step in the right direction?
The most impressive legacy possible? Colonizing another planet seems to be our best shot at that.
Making the next generation proud of us? Well, I mean, look at things we admire the previous generation for. I think the moon landing is up their and it's a hell of a lot less impressive.
Minimizing human suffering? Well yes, then I suppose you don't want to colonize mars, instead you want to start a nuclear war or burn a lot of coal or something.
Maximizing some reasonable average of human suffering and happiness? Well, that's a vague goal so it's harder to argue to, but the benefits are clear, both in longevity of our species and happiness of the members of it. I think it's pretty clear that inspiration and achieving grand goals is very important to human happiness.
Of course, the first thing I said is the issue. What's the expected utility of attempting colonization. Colonizing mars is pretty unlikely to succeed, so it's far lower than the utility of a successful colonization. Still, I think supporting SpaceX is probably a worthwhile investment for our species/civilization/nation/... currently.