My man, I never said it had to be. But where it is, visual communication takes precedent. It took quite a while for me to be able to do this too but, the most accurate building in the world - even if it follows the designated layout - is subpar if it does not clearly show its function in the design.
For the record, I'm not against using the Principia as a base for the roman barracks, I just need to make sure we know our priorities here.
Historicity and aesthetics go without saying, it is the visual communication that tends to go over the artist's* head, so we put emphasis on it.
*sources: me, my head. a lot. Dont make my mistakes