On the down side, what is to stop the cell providers from listing someone's old phone once they upgrade to a new one, even if the old one hasn't been stolen? By turning the old phone into a brick, they could force people to buy new phones to get service.
I can't think the cellphone providers would want to do this since most everyone keeps their old phones as a backup device.
Once your out of contract or fulfilled your contract it I'm sure the providers will not only let you unlock your phone but provide you a clean IMEI from that point on. The thing is those people who dont fill out there contract and skip out on it 1/2 way through if they don't pay the ETF there phones IMEI may/will be put on the block list.
This may also change the current rules carriers have for ETF, they may also add on a prorated $ amount on top of the current ETF to cover the phone subsidy as well. dont pay the ETF and prorated phone subsidy amount? phone gets IMEI banned as well.