Everybody on the Internet advises to disable root login via [SSH](http://en.wikipedia.org/wiki/Secure_Shell) as it is a bad practice and a security hole in the system, but nobody explains why it is so.

What is so dangerous in enabling root login (especially with disabled password login)?

And what is the difference between X symbol username and Y symbol password or root username and X+Y symbol password from security point of view in case password authentication is allowed?