AFAIK LibreOffice doesn't ship with any other font containing a light variant and in my opinion it is very useful for creating appealing documents.
The relevant code seems to be here: http://opengrok.libreoffice.org/xref/core/external/more_fonts/ExternalPackage_opensans.mk
The light variant was deliberately excluded as LibreOffice (and its users) would probably only be confused by it: LibreOffice is works best with two weights of fonts (Regular & Bold) and two styles of fonts (Upright & Italic). Everything else only ends in a mess: e.g. What happens when using the Bold button on a light font? Which fonts are displayed in the font drop-down?
(And yes, I do have enough pro fonts on my computer to know that many weights/many styles are a pain to work with in LibreOffice.)
could we label this as WONTFIX?
Well, we’ve shipped all of the Source Sans Pro weights for a while already and the sky hasn’t fallen. I really don’t see why we shouldn’t ship Open Sans Light.
Old ticket, recommended to close - doing so.