Modern AMD and NVIDIA GPUs are fairly similar hardware-wise, and it is not hard to write OpenCL code that executes efficiently on both. I agree that it is pretty hopeless to write performance-portable OpenCL across entirely different architectures, however.
Sure, but if you go with nVIDIA, you also get access to all the other goodies they distribute (thrust, cudaFFT, cudaDNN, etc) and all the CUDA-compatible stuff other people have written, like Theano and TensorFlow.
It does seem like people have gotten a little more interested in OpenCL lately, but it still lags pretty far behind. As dharma1 says below, AMD seems weirdly uninterested in catching up. If I were in change of AMD, I'd be throwing money and programmers at this: "Want to port your library to OpenCL? Here, have a GPU! We'll help."
