The idea is that with features like this widespread, the probability of a theft leading to the thief repurposing the stolen device will be lower. Therefore, the incentives for thieves – the ones acting rationally within the paradigm of being a thief, at least – will be lower. Therefore, the number of thefts, generally, may go down. A few big ifs here, of course.
I've always felt that system lock was about preventing theft of the device data, not prevention of flipping the device itself. Like tons of other people, I have sensitive apps and data on my phone (banking, cloud services, etc), but unlike tons of people, I take great care to ensure logouts after I am done accessing said apps or data. Even so, a thief snatches my phone, I don't care what happens to the phone itself, so long as the data is safe.
