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Actually, the underlying physics behind the idea is sound. Let's take another planet as the subject, so people won't get so riled up about it. We'll look at VENUS:

Venus has some serious problems with habitability:

(1) It is WAYYY TOO hot.
(2) It's atmosphere is WAYYY TOO thick.
(3) It "spins" too slowly, and in the "wrong" direction.

However, Venus is said to be "Earth's twin" because it is a close match for SIZE and MASS. This means that Venus can offer a human visitor the ONE THING we cannot fake or bring with us on a trip there: EARTH LIKE GRAVITY, vital for normal health and well being.

So, we need to "cool" Venus down, and "spin her up". We can kill both birds with ONE BIG STONE, thrown repeatedly in her direction. By altering the course of a fairly massive asteroid, we can gently "tug" Venus into a cooler orbit, and at the same time gently increase the rate of her spin to match a 24 hour day. Once we get the temps down somewhat, we could introduce aerosol like bacteria to begin breaking down and re-aranging the components of her atmosphere. After about 5000 years of tugging and spinning and bacterial outgassing, WE WOULD HAVE A REAL EARTH TWIN.

Now, imagine humanity with a whole 'nother Earth to live on. Twice the land, twice the resources, and double the chances for our species to live through an astronomical disaster.

Downsides to this plan:
(1) It would cost a lot of money to divert an asteroid
(2) It would take 1000's of years to get to a finished product
(3) If you screw up the calculations or implementations of moving the asteroid, you run the risk of a catestrophic collision

Upsides:
(1) If you are clever about the initial nudging and orbital looping of the asteriod, once you set it into motion, it should largely do all the work on its own, automatically, while we monitor the progress from Earth using telescopes.
(2) We get a whole 'nother planet to play on!

A couple of small nits to pick:

1-The atmospheric pressure on Venus is 91 times that of Earth. An atmospheric probe that was sent was crushed.

2-No moon (tidal necessity)

3-Exceptionally weak magnetic field