When I was hired by Sony Asia Pacific, my first question was to ask them about their stand on the sensor formats after I was told in advance of its full frame camera. The reply was simple. They needed to fragment the market to lower the cost of the cameras and sell more lenses.
APS-C sensors made by Sony were licensed to Nikon, who procured them for use in their cameras. The first full frame sensor from Sony was created to take on the immensely successful Nikon D3, 12 megapixel sensor designed by Nikon themselves. Unfortunately 24 megapixel full frame sensors do not handle low light situations very well but their dynamic range in daylight shooting is superb.
APS-C on the other hand are entry level sensors made to address the market for cheaper cameras. Even then, their capabilities shines when compared to compact camera sensors.
No Reason to go Full Frame if you are a Casual Shooter
This is true. Even Trey Ratcliff of StuckinCustoms.com has adopted the Sony NEX, an APS-C sensor for his current line of shooting. The HDR effects he creates can only be done using a Tripod, if that is the case, then why bother with Full Frame cameras?
In low light or long exposure photography, you have two choices to compensate for any hand held camera shake. Put it on a Tripod or shoot at a much higher ISO. These days, with all the added megapixels on Full Frame sensors, shooting at 6400 ISO can be a huge problem. Something has got to give when you pack so many light sensitive photo sites on a piece of wafer. Noise reduction will be more aggressive and details will be lost in the process.
HDR shooting on the other hand is only done with low ISO, and Trey has demonstrated that on his work. Of course, if you are a Pro, shooting pictures for Stock or Clients, then a full frame camera will offer you insurance in ambient lighting as you dial it up to 3200 ISO.
If you are thinking of using a DSLR, ask yourself first on what you plan to shoot with it. Full frame cameras are a joy to shoot with but are expensive to buy compared to APS-C. If you are a casual shooter, the APS-C will do just fine.