add login and add to sysadmin role by start SQL in single user mode, Security hole or by design?

http://v-consult.be/2011/05/26/recover-sa-password-microsoft-sql-server-2008-r2/ 

by chance viewed this page, and it worked for me to add myself to sysadmin when we lost sa password and no user account has sysadmin right.

Wondering if this is by design, will this be security risk?