Yeah border colour and width per region would be handy but that requires more double buffering of the information because it will be one scan line later when we use it vs when we update it and we can't clobber its working register. It means two extra longs and two more instructions to copy these values over. Plus the extra adjustments to the streamer commands and restoration would add more. Doing it once for the display helpfully uses far less COG resources.
I sort of originally envisaged the optional border to be used for managing overscan type problems for screen centering (where it would often be black, but still had the ability to get coloured according to the overall application preference/style, so they need to be active pixels), or for retro C64-type screens in text modes - though I think that platform was actually able to change the border colour per scanline IIRC.