XZ compressed packages have been used for a while by various distributions, including the Gnome repositories, but Ubuntu's developers are still reticent about switching to another compression.
Gzip is still a good compression algorithm, but XZ has made it clear that it is the future. It provides smaller packages, but there is more to this problem than meets the eye.
According to phoronix.com, the problems of adopting XZ compression for Ubuntu 13.04 has been raised at Ubuntu Developers Summit, but no conclusion has been drawn yet.
Even though the compression factor is a very powerful argument, Canonical thinks the trade-off may not be sufficient.
As it stands, smaller archives would account for a reduced download time, but it also means an increase in decompression time, on less powerful hardware configurations.
Canonical just has to do some more testing and determine whether the saved time on download is smaller than the increased time of decompression.
If so, Ubuntu 13.04 will feature XZ compressed packages.