What I mean is that they've already decided to mug you, they'll take that phone whether it's locked or not, whether they know it or not ("maybe we can get our computer guy to unlock it!").
On the other hand, having the phone send out photos and GPS coordinates is a much better idea, and it's been proven to work.
Sure, in this specific instance, they may do that. However, after stealing phones that turn out to be unusable (and pretty much something that will lead the police right to them), they will stop mugging people.
They don't know it now, but eventually people will figure out that they're impossible to fence.
You're lowering the reward while holding the risk constant. As technology makes the street value of stolen items lower and lower, eventually street robbery will be less attractive.
On the other hand, having the phone send out photos and GPS coordinates is a much better idea, and it's been proven to work.