As per summary.
not that big of a deal in practice since xbmc requires other packages to be built with png support ... http://sources.gentoo.org/media-tv/xbmc/xbmc-10.1.ebuild?r1=1.5&r2=1.6 http://sources.gentoo.org/media-tv/xbmc/xbmc-9999.ebuild?r1=1.84&r2=1.85
For the record, it is important as otherwise it wouldn't get listed in the revdeps that I use to test libpng versions.