It's what people asked in the other thread.
At least, aliens won't shoot animals for no reason, the player won't have to wait for 20 animals to move every turn, cattle won't randomly appear and move in buildings, and XCOM agents won't be blocked when a chicken is guarding a door.
Also, long animals like horses and cows work very bad as units : as you said, cylinder hitboxes, and forced to be modeled as 2x2. As tiles, they can be 2x1 with mostly correct LOF, and mapmakers can place them logically.