I think it is true that if nothing is listening on the port, then the packets will be dropped by the OS. However, you may have services you are unaware of running, or malicious software which has snuck in, or a running application which would be listening on the port if it was open. Perhaps you want to run a webserver on localhost for testing, and by keeping the ports closed you block remote access. For these reasons it is generally regarded as bad practice to leave all ports on a server open to the public.