You write OpenCL if you want to target anything other than AMD/NVIDIA/Intel. If you're writing code for an embedded application (with some heterogeneous core), or for a mobile application, you absolutely have to write OpenCL code, as there's no alternative. OpenCL is shit, but it's cross platform shit.

If your aim is to get 100% performance in a GPU heavy cluster, then sure, you're going to need to write CUDA code, and buy some NVIDIA GPUS, however there are a lot of applications which run in entirely different environments which _only_ support OpenCL.