There are many functions and levels where OpenCL may not be the proper method to get the best results. OpenCL still lacks a vast array of functionality for GPU-based 3D rendering, for example. Until it catches up, which may or may not even happen at all, CUDA is the only viable cost-effective solution. Not that there's a lot of GPU rendering going on in the industry yet, but it's expanding for certain as Nvidia's solutions take hold in various pipelines.
Thus, AMD is sometimes not even an option. Nvidia's own mental ray "iRay" (yes, I also hate the name) is CUDA-based, and there aren't many alternatives in the industry. And in some cases, there really doesn't need to be. AMD makes great cards too, but it would be impossible to recommend one for 3D creative content (Maya, 3DS Max, every CAD application) with any degree of honesty.
Don't get me wrong, I'm not a brand fanatic. I do tend towards AMD CPUs, but Nvidia GPUs are the cornerstone of any nutritious CG artist.