This would be hugely useful for remotely deployed ship-based (operating near shore) sensors which use cell networks to send back readings. For security and environmental reasons, the sensors are sealed boxes with no physical access to what's inside, including the SIM. What happens if the device is moved to another carrier's coverage area, or another country? What happens if the current carrier goes under, or they jack up their rates, or change their roaming policies? Right now, a carrier going under would brick the device.
Additionally, think beyond phones, as TFA implies. Physical access to a SIM is a security risk in itself. I would rather restrict phyical access to the SIM, and have password restricted access to a reprogrammable SIM than have my sensor drop off the air, or start sending readings somewhere else altogether.