Yes, much easier to colonize. We are actually doing this in a small way, using deserts to grow crops, with the salt water greenhouses, so I think you can say that not only should we do it, but we already are - though not so much in the Sahara, this is an Australian desert project. The sea water is used to make water through the sunlight in the desert, and cool down the greenhouses.

These ideas could be used to reverse desertification in the Sahara desert and other deserts. This is how it works:

Diagrams by Raffa be from wikipedia

It not only lets you grow crops in the greenhouses - it can also help make the surrounding areas more habitable, so you’d get trees and crops growing in an area around the greenhouses as well. Doesn’t extract anything from desert aquifers, rather, it adds to them.

Sundrop farms have a large area set out for greenhouses like this now, in the middle of a desert, so this is taking off in a big way in Australia. Early days yet though.

This video just shows the greenhouses, and when they go inside in the video there is nothing growing there yet, not sure why, maybe it is a new installation, but it shows how it’s quite big in Australia.

There are many countries working on reversing desertification Israel does a lot of reversing of desertification.

One of the worst areas of desertification is the southern edge of the Sahara desert which is creeping south. So the first priority there is to stop the spreading desertification - then to reverse it.

There is a similar project underway there now too, the Sahara forestation project, again using seawater greenhouses.

Technologies - Sahara Forest Project

There’s also the idea of floating sea cities, a similar idea again, but now just floating in the sea. However this is at a much earlier stage, more of a paper project at present. Still it is very similar to the Mars colony ideas, but much easier to do and floating on the sea.

The Seasteading Institute | Opening humanity's next frontier

This gives far more food / living space for the cost, compared with the billions of dollars to set up a few people in a space habitat, and far far easier to build a greenhouse in a desert on a planet with abundant sea water, and breathable air, than to do it on Mars.

Going into space is not the optimal way to create a larger habitable area for humans in our solar system at present, as much of Earth is uninhabited - and not areas that are in need of conservation either - of course some desert areas are of great ecological interest but there are plenty of places where the desert is not of especial interest, and where colonization would be beneficial.

There’s the surface of the sea also. We could have sea cities covering much of the seas if we really need more space for people to live. If self sustaining colonies are indeed possible on Mars, they are certainly possible and much easier to construct, floating in the sea. I mean ones that only use the sea water and the air and nothing else, with a few imports from Earth - that would be the equivalent of a Mars habitat. No need for fishing or anything else, just air, and sea water, and the materials to build the city originally, and some imports, and you are in a far better situation than you could ever be on Mars.

The air is breathable, no need to generate oxygen or to scrub the air of harmful gases, the Earth as a global system does that for you, just open your windows or make sure you have a bit of ventilation in your homes. There is no need for several meters thickness of cosmic radiation protection. You don’t need to wear spacesuits to go outside to repair the habitat, and there is no need to hold the breathable air in against tons per square meter of outwards pressure - hard to beat that.

Also, it would have minimal impact on sea life if done that way.

I think there is a case for sending humans into space, I think the Moon is the obvious first destination. Things we can do include

• Detect and deflect asteroids that could impact on Earth
• Supply satellites and other spacecraft with fuel and other components produced on the Moon or from asteroids - for much less cost - possibly - because you eliminate the need to launch the materials to orbit from Earth
• Move some of our heavy industry into space
• Scientific research and astronomical observations
• Exploration and adventure
• Tourism, could be a major thing for the Moon.
• Manufacturing electronics on the Moon - it’s a high grade vacuum which makes some processes easier to do there than on Earth.
• Solar power. It can actually make sense to do solar power in orbit, because you can use huge thin film mirrors weighing only grams per square meter, to focus the sunlight, onto a central thermal power generator and then safely beam back to Earth using microwaves. Also can be possible to create solar panels on the lunar surface

I think there are many good reasons for going into space with humans. But this is not one of them in my view.

See also my Case For Moon First

As for the idea we have to go to Mars to go multiplanetary, no, I don’t think we need to do that at all. What we have to do is to protect and save Earth, the only place easily habitable by humans in the solar system. I think that a rush to send humans to Mars would

• Spend trillions of dollars, eventually hundreds of trillions if they do it in earnest, on trying to make Mars habitable to humans, that would be much more beneficially used to protect and help Earth. It’s not going to work at all in the near term and probably never - the most optimistic estimates by the Mars society suggest a thousand years involving mega technology such as mining cubic kilometers of fluorite ore on Mars and using 500 half gigawatt nuclear power stations to convert it into greenhouse gases - and even then there is a lot of question about whether that would work - and how likely is it that we would keep up such a project for a thousand years when it is much easier to live on Earth? And it needs to go on for 100,000 years to get all the carbon sequestered for an oxygen atmosphere, nitrogen needs to be sourced somehow, many cycles somehow got working on Mars in a different way from Earth, and lots to go wrong and needs mega technology into the indefinite future to keep the planet habitable (barely) if it works.
  
  If you read Kim Stanley Robinson’s “Mars Trilogy” then that is fantasy / science fiction with a lot of fudging of the numbers for the sake of a good story that plays out in generations instead of millennia. In the Earth’s evolutionary past of course it took many millions of years.
• Also, by introducing Earth microbes to the planet, interfere with the process of scientific study of Mars which has potential to lead to a discoveries of biology based on different principles form Earth life or better understanding of early stages of how life can evolve from non living chemicals that could be as momentous as the discovery of evolution or the helical structure of DNA. And very unlikely that at this stage we know enough about which microbes to introduce when, never mind introducing a random grab bag of whatever happens to be in human occupied spacecraft, if we do decide to do ecoengineereing of Mars. I think it will be a fair while before we are mature enough to understand the consequences and know how to do it, to embark on a career of planetary ecoengineering as a species. We can do it small scale first with Stanford Torus type habitats or lunar caves / city domes, projects that we can complete in a more realistic decade, rather than a hundred thousand years.

It would take a while to explain all that in detail - so see my Wait, Let's Not Rush To Be Multiplanetary Or Interstellar - A Comment On Elon Musk's Vision