Passwords that are unguessable in a certain amount of time, I'll buy. The guy trying to steal your bike trying different combinations has a finite amount of time he can work before someone sees him and becomes suspicious. His chances of success that way would be MUCH greater. OK, I'll admit things are better these days, longer passwords, more possible characters, delays during login attempts, etc. BUT, the point is that someone who wants to remain invisible and is very patient may very well break ANY password.
Passwords alone are insufficient.
You HAVE to have some other mechanisms to slow down what a password guesser will be doing and hopefully some mechanism for spotting what's happening and reporting it to someone before things progress very far. Not bad for an evening's work. This group of 169 has shown up in the last 12 months. Most of the passwords were either the person's first name or his login name, all lower case, forwards or backwards. At the rate of over a hundred accounts per evening, it certainly wouldn't take too long.