This is a good point, but do note that the topic was decompression speed. bzip2 is pretty good in terms of compression speed and ratio, but performs very badly at decompression.
Just for some rough figures, I'm decompressing the Linux kernel 2.6.32 source tarball, on my quad-core Phenom II system:
pbzcat, four threads, takes 4.1 seconds of wall-clock time (15.6s CPU time).
xzcat, single thread, takes just 4.7 seconds.
zcat, single thread, takes 2.3 seconds
So, parallel bzip2 decompression will probably beat gzip at 8 cores, whereas XZ would be on par with just 2 cores. While XZ is slow at compression, it will definitely beat gzip and bzip2 in parallel decompression.