Here in Australia we're going mad for certifications. I had a friend who looked after kids in an after school program. A minimum wage job, but one that she loved. She'd been doing it for nearly 20 years. The government mandated that she needed a $2500 certificate in order to continue to do the job she'd been doing for two decades.
I don't believe there was anything that this certification could have taught her that she didn't already know. Heck, she should have been the one teaching the course. Instead it was being run by an organisation that specialised in running these kinds of certification programs, but certainly didn't specialise in the subject matter.
If we were to mandate certification for managers, my concern would be that - at best - it would end up as just another piece of paper. At worst, it would actively exclude the people who were most qualified and competent, like it did with my friend.