A thermonuclear war and the ensuing nuclear winter would shift the situation quite a bit. Which is not that far-fetched, we have the capability and have been dangerously close a couple of times.
Earth might still have stronger fundamentals, but it is good to have options. And, perhaps more importantly, technology developed in pursuit of becoming a multi-planetary species might very well help here on earth too - both in our current development path and in case of disaster.
I generally agree (and I'd add: magnetic field, more sunlight - not just for temperature, but also power generation), but come on, Mars does have gravity :).
Even if we all had to live in bio-domes because the planet was so polluted, it would _still_ be a better option than Mars, just by virtue of the fact that we have gravity and an atmosphere. On Mars, a cracked bio-dome means everybody dies. On Earth, you could probably just go fix it.
Earth has: