Bat guano represents a biohazard with the potential to cause severe illness. The droppings turn to dust when they dry and the small particles and fungi present can be breathed in if they enter the air, especially if they are able to disperse through your home.
The droppings will also cause unpleasant odors and stains that continue to linger after bats are gone. This is why cleanup and restoration are as much a part of bat control as removal.
The longer an infestation has been present, the more droppings are likely to build up in the space they roosted. These larger accumulations of bat guano should be addressed by professionals whenever possible, even if you were able to complete exclusion on your own. This is true as well for droppings that are difficult to reach.