It is definitely a bad thing: as a postdoc you have to constantly uproot your life chasing the ever-elusive permanent contract so good luck rebuilding your social circle from scratch every five years. And God help you if you're married or (the horror!) want to have children.

It's also not good for the Universities, who are constantly losing institutional knowledge and having to retrain people from scratch. Did you offer a kickass course on Neural Networks last year? Well, too bad, the guy who offered it is now gone. But here's a fresh guy with a PhD in economics, and that's just as good.