In reply to Thomas Ehrig.

can not account for the uniqueness of each and every theme. Its just not possible

If your theme requires specially formatted data to make it function, then it is not a "theme".

What happens when I take my years old site and switch to your theme? If it looks like crap, then that's a bad user experience. Any theme should be able to handle a basic site correctly and beautifully. Those that can't shouldn't pretend that they can by faking the preview data.

Just my opinion.