[Guest]

With a curved focal plane corners are not unsharp. They are just sharp at a different plane. 

 

Some logic: one usually focusses on the subject or a main attraction in the scene. What you are proposing is intentionally back focussing the main subject or intentionally front focussing the subject in order to get the corner focal plane closer to the subject focal plane. Does that sound like a good idea?

For some, who want to have the same sharpness across the whole field, even taking into account stopping down, it may be what they want to do.