Imho this could (should) be an experimental feature, turned on/off in makepkg.conf.

I have modified my packagers and now using pigz instead of gzip and pxz instead of xz for a year without glitches and errors.

I think, both ways are simple and reliable, i prefer to change the tools and leave pacman and makepkg untouched.

And it speeds up compressing and decompressing a lot on my AMD 965 ;)