Right now we have two types of theives to worry about opportunistic and career. Today if you find an iphone laying about, with little more than a DFU you have a blank phone that retains all(most?) resale value. If you found a few hundred dollars lying around it might be hard to turn it up, but that will all change with an activation lock.
By creating an activation lock you drastically reduce the oppotunistic value of the iphone. If a random person finds a iphone laying about there is a high risk that it will have a drastically reduced resale value over what they have today. Rather than an easy cash resale on CL or ebay the cash value is risky and you could part it out, but that means holding on to it which increases the risk as well.
This doesn't stop a career criminal who can and will assault you for the password, but that is a level that you would never be able to stop until the carriers intervene, this at least tips the scale dramatically in the favor of the rightful owner. It is unlikely that this will turn opportunistic thieves into criminals that would assault you.