Is it worth dropping money into teleconverters? Or, should that money be spent on a Dx camera instead? I'd say the Dx camera. Below I show why...
A Dx (crop sensor) camera has a crop factor of 1.5 for Nikon and 1.6 for Canon. What does this mean? Well, it means that a 200mm lens on Nikon Fx will be the equivalent to a 300mm lens on Dx. It doesn't make it a 300mm lens, because the compression will not be the same. But the crop will be.
A teleconverter will give you the 1.4, 1.5, 1.7, 2.0x reach, however the trade off will be that you'll be losing a proportionate amount of light with them. So you would have to up the ISO, open aperture or slow the shutter to compensate.
Plus, you can still throw a teleconverter on Dx! Below are 2 examples for Dx and Fx:
Dx - 200mm lens with a 2.0X converter would be equivalent to a 600mm lens with a 2 stop light loss (4 times less light)
Fx - 200mm lens with a 2.0X converter would be equivalent to a 400mm lens with a 2 stop light loss (4 times less light) To get it to a 600mm lens equivalent, you'd have to stack a 1.5X converter with the 2.0X. So, 1.5X would lose 1.5 stops of light. Plus the 2.0X. It would be a total loss of 3.5 stops of light.
In conclusion, and this is my opinion, I'd get Dx camera like the Nikon D7100 over a teleconverter any day. I'd only consider a teleconverter IF I had a Dx camera, AND needed the extra reach permitted by Dx crop factor, AND couldn't physically get closer to the subject.