Practically speaking, this won’t make a huge difference to desktop developers right now. Because S3TC is a required part of the Direct3D specification and all desktop GPUs support Direct3D, S3TC has been a de-facto OpenGL standard for nearly 10 years now. And because few developers will target OpenGL 4.3 right away, that won’t change. But this means that developers targeting 4.3 do finally have a choice in texture compression, and developers doing cross-platform development with OpenGL ES can use the same texture compression format in both cases.
It’s worth noting though that just because a GPU “supports” ETC doesn’t mean it has hardware support. NVIDIA has told us that they’ll be 4.3 compliant, but they’re handling ETC by decompressing the texture in their drivers before sending it over to the GPU in an uncompressed format, and while AMD wasn’t able to get back to us in time it’s almost certainly the same story over there. For ports of OpenGL ES games this isn’t going to be a problem (dGPUs have plenty of high-bandwidth memory), but it means S3TC will remain the de-facto standard desktop OpenGL texture compression format for now.