"Locking" can mean many things. Starting from limiting the choice of the network provider, and ending with signed kernel limitations and locked bootloader. What we are concerned with is the later. I.e. will there be a way to unlock those on the device which is "regular" or one will have to use special developer device for that?

Simple use case where it might matter. Imagine someone creates a kernel module for supporting certain scenario (like USB host mode or what not). Open mode / device allows installing such module. Will the user of the "closed" mode able to unlock the device and use it too, or such user will be forced to buy another device to use it? That's the main point IMO, and concerns about fragmenting the user base.