Do you consume a lot of hi-quality scans from museums/archives/libraries? This is the common standard. The LoC distributes all of the high quality scans in tiff format:

http://lcweb2.loc.gov/service/pnp/cph/3b40000/3b48000/3b4880...

http://lcweb2.loc.gov/service/pnp/cph/3b40000/3b48000/3b4880...

Why do so many projects still distribute gz or bz tarballs when xz is the best?