The main issue is that fbdev has been designed with the implicit assumption 
that an fbdev driver will always own the graphics memory it uses. All 
components in the stack, from drivers to applications, have been designed 
around that assumption.

We could of course fix this, revamp the fbdev API and turn it into a modern 
graphics API, but I really wonder whether it would be worth it. DRM has been 
getting quite a lot of attention lately, especially from embedded developers 
and vendors, and the trend seems to me like the (Linux) world will gradually 
move from fbdev to DRM.

Please feel free to disagree :-)

I would disagree on the "main issue" bit. All the graphics cards have
their own formats and cache management rules. Simply sharing a buffer
doesn't work - which is why all of the extra gloop will be needed.

Alan