@Lord Nightmare - Very nice!  Thank you for this "rescue" !  :^)

As a 5100 user/programmer long ago, and having had access to the 5100 service manual at the time, I can say that at least one reason for the no-consecutive-dots rule was the appearance of the font on the screen itself. The effective resolution of the screen was 640x192, non-interlaced (with 10x12 character cells, giving 64 characters by 16 lines). The scan lines were separated by about as much space as they were thick, so a solid horizontal line would have looked twice as bright as a solid vertical line. By only ever activating every other dot, the horizontal lines would have the same apparent intensity as the vertical ones, making for a much more pleasing display. The printer might have been another reason, but I quite clearly remember the above explanation in the video display section of the service manual.