If you don't specify that protocol, though, you have three scenarios:

1. only the local host has the rsync binary 2. both local and remote hosts have the binary, but neither runs them as a daemon 3. both have the binary and the remote runs as the daemon

In #1 you end up using SSH anyway (unless there's also no SSH binary). In #2, a malicious server binary could attack you. In #3, a malicious server binary could attack you.

Also, many of rsync's features rely upon both sides having the binary.