It's quite effective, tbh. Stealing phones is still a way to net loads of money if you know a buyer who doesn't give a damn about the source. Hard-reset the phone so it appears clean and off you go.

The only vendor that actively prevents the usage of stolen phones is Apple, because the owner can remote-lock it even if the phone gets wiped via iCloud.

The ideal way would be if network operators cooperated by maintaining a list of "supposedly stolen" IMEI codes and alerted police if a matching phone tries to log in into the network.