We have vents on our ceiling but they're not connected. Can we get rid of them, just drywall over them?

Our heating and A/C comes through floor vents. There are vents on the ceiling but they just go up into space, not even an attic. The roof has peak vents and the space up there has some shredded paper insulation. Is there any reason NOT to get rid of these vents and just drywall over them, maybe texturing so that they're like the rest of the ceiling?