Grills are for aesthetics and protection. On the ceiling, protection is rarely an issue. So if you prefer the way it looks without the grill, I can think of no good reason to install one.
Another possibility to conceder is, if you find a grill that better fits the theme you want to create in the room, use that one instead.
Another possibility to conceder is, if you find a grill that better fits the theme you want to create in the room, use that one instead.