In my not so humble opinion, social-democratic parties are backing this idea due to their highly dogmatic doctrine of full employment, which they can't let go of.

Full employment is just not a viable proposition in any capitalist system (whether French, Anglo-Saxon, or Rhine 'model'). Unless you dump your long-term jobless into the sick/disabled category and increase temp work, as the Dutch and British are wont to.

Your last question is, like, the mother of all questions. Let's have a debate.