Just like I don't want snail mail carriers opening my letters (even if it is strictly for the purpose of deciding whether or not they are important for speedy delivery).
There should be a protocol in which as much as possible is hidden. Even things like port numbers. From the traffic pattern it will usually be obvious whether it is bulk or interactive and such, and it has to be addressed, but beyond that, what protocol it is using, what port number and all that should be shrouded from prying eyes. If I have two connections going to the same host, it should not be possible for a wiretapper to tell which packets belong to which connection only "host A sent some blob to host B".