Another issue with a teleconverter is transmissive light loss. A good 2x converter usually has seven elements, and a cheap one has four elements. A seven element converter will often have a half stop light loss in addition to the two stop loss due to doubling the focal length while maintaining the same physical aperture diameter, and if you use non-TTL auto flash or a handheld meter, you need to account for it. TTL metering will take of it.
One situation where people might combine an extension tube and a teleconverter is bird photography, because you are often photographing small subjects with a really long lens that isn't long enough, and if you're lucky, you can be closer than the near focusing distance of the lens, so if you've got a 600mm lens and want to photograph something the size of a sparrow, you'll need a teleconverter, and if you can get within 12 feet, you'll probably need an extension tube.