Apple put security into iPhones to make them much harder to sell after they have been stolen. iCloud Activation Lock makes it very hard for a thief to resell a stolen phone, thereby making it less attractive to steal in the first place.
So, the thieves that I was in jail with told me they continued to steal the phones then just took them all to the phone repair stores who would break them all down for the spare parts and give them cash. They would then use all the stolen spares to repair other people's phones.
Now that the parts are serialized this channel is closed and it again makes it less attractive to steal the phones.
So, I for one am OK with the serialization, BUT there needs to be a balance. I realize Apple now hands out repair tools and instructions, with a system to reserialize your authentic parts, but what to do with a mountain of half-broken phones that could be legitimately cannibalized for their working parts?