Is Android Device Manager really useless?
There is two parts to it,


Cell phone is lost : Lets assume its lying somewhere or someone found it. In this case you can use the Device Manager to locate the location.
Cell phone is stolen: This guys are smart, they know the technology, they know the hacks. They will switch-off with cell phone as soon as they can. They will wipe out our data and most probably brick the phone. Not sure, but I think they even change the IMEI number to make it completely untraceable. In this case Device Manager or most security software is useless.