If national governments can launch their own payloads without having to go through ESA or ArianeSpace it'll undermine the latter's position. They may even become competitors.

Italy accused the French-based ArianeSpace of deliberately taking launches away from the Italian-build Vega rocket in favor of the French-build Ariane rockets. They've now more of left ArianeSpace and are selling the rockets themselves and using ESA's infrastructure to launch them.