And this is the other value of the EUPL: the public sector is conservative and "EU" name gives them a safe feeling. (So, adoption is slow but real, and we're surely getting more free software than if no EUPL existed.) If a government body is approached with FUD about free software, we can point out that they can use the EUPL. It's unfuddable.
And by extension, having GPLv3 in the Appendix is probably also good for the image of GPLv3 in the eyes of the public sector.