To delete these cookies again just click on "Log Out", next time you go to the scripts you'll have to log in again.
To answer the question - cookies are everything else than a security-risk. The passwords are stored in a crypted way on the server and in your cookies. Nobody else has access to them. If you don't use them you do not only have to login every time you want to login, your computer also has to send the password to the scripts and they back to your computer every time you click on a link on the Schlabo's Scripts admin-pages.