I would only really worry about them if they attack someone, because the only time they do that is if they have rabies.
But in a building they will create a huge problem. I worked in an office building where one of the tenants moved out and some windows were damaged. The floor went unleased for more than a year, but when our company attempted to lease it out for expansion we quickly found that the amount of guano in some of the rooms of that floor was insane. The building owner had to bring in specialist to get rid of all the bats and deal with the guano.