Louis said:
"If you really want to get out there and colonize space, you need religious fervor. Of course, our obsession with science and technology has reached religious proportions today. So there may be enough nutcases around who would actually try it, although, from a utilitarian point of view, it's absolutely unnecessary."

J-- Are you absolutely sure that colonizing other planets is "absolutely unnecessary"?

I agree that this Earth should be "preserved" in a condition that will support human species for countless eons.
But, I also believe that there is the possibility that some unpreventable or unforeseen catastrophe will wipe out all intelligent life on this planet. If such an "unthinkable" event CAN happen, then is it not "wise" to have humans on at least one other planet?

The "unpreventable event" could be the collision of a large asteroid or a comet with the Earth.
Unfortunately I can not "foresee" the "unforeseen", but I can point to a potential problem that has not been thoroughly studied. Let's consider the following (fictional) situation.

We will use increasingly more thermonuclear devices to generate power, such "controlled chain reaction" will increase the radioactivity in the surrounding environment, the "environment" includes the Earth's atmosphere.
Radioactivity is "energy" and such energy has the ability to "speed up" the motion of molecules. The "speeded up" air molecules (being lighter) will migrate to the upper reaches of the atmosphere (exosphere). When these "speeded up" molecules have enough kinetic energy they will escape from the Earth's gravitational influence and will be lost into space. Such loss of air molecules will decrease the atmospheric density and pressure. When the atmospheric density and pressure decrease, the wind velocities will increase, (storms will become more violent) more sunlight will reach the surface, surface water will evaporate, lakes will dry up. When enough atmosphere has been lost, then the Earth's internal pressure will cause "bulges" in the weak areas of the Earth's crust. Volcanic activity and earthquake activity will increase with devastating results to the surrounding areas.
If the atmospheric pressure becomes extremely low, it will become impossible for humans to live on this planet.

There may be other "unforeseen" events that can happen. All problem areas that we can imagine should be studied.

In my opinion, it is worth while to explore Mars and other planets and also to figure out how humans could live there.

Johannes