the xmlto script sets the default XSL directory thusly: : ${XSL_DIR=/usr/share/xmlto/xsl} there is no such directory that is installed as part of xmlto (perhaps there used to be?). it doesn't hurt anything to leave that as the default, but it does seem somewhat pointless.
I will not fix with the patch and I will not create new tarball because of that - as you said - it doesn't hurt anything to leave that as the default. Anyway - it could(and maybe should) be fixed in next upstream release, so I will ask upstream about that issue. So WONTFIX for now/but could be fixed in future...