This function should take cubemap textures too. Then we would never have to use glFramebufferTexture2D.
I'm not sure how this follows. FramebufferTextureLayer deals with
3D and array textures, so the non-array targets supported by FT2D
would not be supported without that command. I think it's pretty
unlikely the ARB would want to remove existing functionality but
if you want to file a specific RFE as to how FTL should support
cubemap textures, I'll pass it on.
I'm going to WONTFIX this - if you have a specific proposal please file
it as a new bug.