A lot of phone thefts in big cities like NYC/Chicago go like this:
Person is on the train, engrossed in their facebook news feed (and probably have their headphones on so as to be further zoned out). Train pulls into stop and doors open. Thief rips phone out of person's hand and dashes out the closing doors. By the time the victim or anyone nearby recognizes what just happened, the doors are closed and the train is pulling away (and to an outside observer on the platform, it just looked like a guy who forgot it was his stop and dashed to get off the train). Less pro thiefs may not have the timing down, but they can probably run fast enough to still avoid getting caught.
If everyone knew that that phone would become worthless, these types of thefts would calm down. You might have a phone to play with for an hour (and if you were savvy, you might try using the linked email to break into some financial accounts), but the sketchy guy in the ice cream truck with a "cash for phones" sign isn't going to want soon-to-be bricked devices.
As an aside, if I were a robber, I am not sure if I would take the phone or not. Maybe take it and toss or break it (especially if it will be bricked remotely). The last thing I would want after robbing someone is something that is so easily tracked. My movements after the theft might be recorded and the phone might be hard to sell since they are much easier to check the history on than cash or random jewelry (and I would have to turn it on show any potential buyer that it was working).