alpha and refraction are NOT the same. Naeq example is good.
The real question is : is that usefull for compositing ? It's not. The composited background will not be affected by IOR so it's useless.
You seem to be unable to see further than the boundaries of your own workflows. There are quite a few compositing packages, such as fusion, which allow you to import geometry and put realtime GPU computed refraction on it, then use that to refract the background added on postprocessing and mask it out by alpha which takes refracted environment opacity into account. And there are some pipelines that rely on it.
The exact same thing that's happening here in Corona is happening also in V-Ray, when you set refraction to affect alpha channel. And V-Ray has been used for quite many feature films in quite many VFX pipelines, and they don't get people complaining about this behavior, because it works as intended.
I am not implying that everything V-Ray does is right, but in this particular case, it is.