Out of all the free and open source licences which are available, there are two which are disproportionately chosen by FOSS developers when licensing their software. Those two are the GPL and the LGPL. Of these, the GPL is disproportionately favoured over the LGPL.* If there are issues with GPL implementations then there are IP issues with OOXML. Any assurance that excludes implementation under these licences is just cause for the FOSS community to voice concern.