But telnet and rlogin are not the only two programs that increase the risk to a site. Any other program that allows incoming connections and does not have the ability to encrypt it's data is a risk.
Some examples are ftp, pop and older versions of sendmail .
Sniffers can easily pick off passwords transmitted in plain-text from any of these programs. And the problems with anonymous ftp could easily take up a few paragraphs. In addition, there are still vendors that ship their software with non-passworded accounts by default, which is yet another risk. However, this question was explicitly aimed at telnet and rlogin so let's just stick to those for now. Telnet is used to make connections between machines on a network.