TCP/IP network administration has never been simple. And yet, many of us remember a time when connecting a host to "the network" meant one's local area network (LAN), which itself was unlikely to be connected to the Internet (originally the almost-exclusive domain of academia and the military) or any other external network.
Accordingly, the threat models that network and system administrators lived with were a little simpler than they are now: external threats were of much less concern then. Which is not to say that internal security is either simple or unimportant; it's just that there's generally less you can do about it.
In any event, in the old days we used telnet, rlogin, rsh, rcp, and the X Window System to administer our systems remotely, because of the aforementioned lesser threat model and because packet sniffers (which can be used to eavesdrop the passwords and data that these applications transmit unencrypted) were rare and people who knew how to use them were even rarer.
This is not so any more. Networks are bigger and more likely to be connected to the Internet, so packets are therefore more likely to pass through untrusted bandwidth. Furthermore, nowadays, even relatively unsophisticated users are capable of using packet sniffers and other network-monitoring tools, most of which now sport graphical user interfaces and educational help screens. "Hiding in plain sight" is no longer an option.
None of this should be mistaken for nostalgia. Although in olden times, networking may have involved fewer and less-frightening security ramifications, there were far fewer interesting things you could do on those early networks. With increased flexibility and power comes complexity; with complexity comes increased opportunity for mischief.
The point is that clear-text username/password authentication is obsolete . (So is clear-text transmission of any but the most trivial data, and, believe me, very little in an administrative session isn't fascinating to prospective system crackers.) It's simply become too easy to intercept and view network packets.
But if telnet, rlogin, rsh, and rcp are out, what should one use? There is a convenient yet secure way to administer Unix systems from afar: it's called the Secure Shell.