Why waste time and money figuring out the interface for an unknown screen instead of just buying a known one with a datasheet?
Because the 'unknown' ones used in high-volume consumer electronics are often available a lot cheaper and easier to get hold of as spares than ones that come with datasheets.
However issues like connector non-availability , like the ipod one
and not-quite-compatible clone controllers (like some batches of the Sparkfun 6610 ones
) can spoil the party.
However it really isn't a big deal to reverse-engineer LCD protocols if you have the device that the LCD came from - although some people seem to get rather over-excited about it. The trickiest bit is usually breaking out the fine-pitch connector while keeping the product in a sufficiently working state.