In recent years the use of telnet has dramatically reduced with the introduction of the far more secure ssh.
Note that while telnetd is rarely if ever used anymore, the telnet client itself is still extremely useful for troubleshooting many server applications; for example, one may use telnet to interact directly with SMTP, HTTP, IMAP, or POP3 daemons to allow more effective troubleshooting. (See Daemons, testing via Telnet for examples.)
-l <username> -- used to specify the username to use when logging into the remote machine. -n <filename> -- used to create a trace file of the telnet session.
[light@splat ~]$ telnet insecure.host.com Trying 10.0.0.1... Connected to insecure.host.com. Escape character is '^]'. Password: [light@insecure ~]$ logout Connection closed by foreign host.