GeoThermal energy is another use of the Sun's radiant energy.  Just because we're not turning solar rays directly into energy doesn't mean GeoThermal is not making use of the same energy.

As an example, think of a GeoThermal heating and cooling system.

Well?

Eventually, specific areas can cool down, and there's really no way to "reheat" the earth. There is also concern that where geothermal energy is used for large projects, such as power plants, that the method that's commonly used of injecting superhot water back into the lands can adversely affect the stability of the region.

To put it simply: It's taken millennia for the earth to store all that solar energy. If you use up all that heat in a few decades, there's no telling what the long term consequences will be. We're much better off eliminating the middleman and harnessing solar power directly.