Watched a show about Mars on the DVR last night that said that Mars, being smaller than the Earth, cooled sooner. So maybe the same thing simply hasn't happened to Earth yet.
Maybe, but Earth has a moon to exert significant tidal forces. These affect not only the oceans, but even the fluid in oil reservoirs, and the planet's liquid core. It's safe to say that has some effect on the planet. Whether Mars had some sort of liquid core, (you'd think there would be some remnant magnetism), is hard to say, but it didn't exert the same effects, possibly related to the absence of a moon of significant ehough mass.
The magnetic field not only orients critters (migratory and other navigation), but produces field effects which shield the planet from ionizing radiation (Van Allen Belts).
A hot core (likely nuclear decay ) keeps the middle layers in flux, shunting the surface features around (plate tectonics), and affecting the planet surface as well as atmosphere and weather/climate.
There are a lot of wonderful and relatively (within the scope of our knowledge at this point) unique features of this planet that contribute to it being habitable.