Nope. There are other instances where you may want to find your phone without letting the person using it that you are locating it.
1) Let's say you work in an office and someone is taking your phone and doing something with it. If the person is alerted that the phone is locked, they will put it down and you will never know who did it. If you can locate it and take pictures of the user without locking it, you find out who is doing it and can confront them about it.
2) I travel, and there may be cases when my wife wants to see where I am or if I am stuck in traffic. I do not mind if she checks my location, but I do not want my phone locked.
If the phone is truly stolen as you say in your example, that is no problem if there is an option to lock the device. There would be no difference from the time it is stolen to when the user locks the device compared to the way it works now.