http://onemansblog.com/2007/03/26/how-id-hack-your-weak-passwords/
Very informative article on how easily weak passwords are broken.
This table from the article should be fairly convincing as to why you would want very good passwords:
| Password Length | All Characters | Only Lowercase |
|---|---|---|
| 3 characters 4 characters 5 characters 6 characters 7 characters 8 characters 9 characters 10 characters 11 characters 12 characters 13 characters 14 characters |
0.86 seconds 1.36 minutes 2.15 hours 8.51 days 2.21 years 2.10 centuries 20 millennia 1,899 millennia 180,365 millennia 17,184,705 millennia 1,627,797,068 millennia 154,640,721,434 millennia |
0.02 seconds .046 seconds 11.9 seconds 5.15 minutes 2.23 hours 2.42 days 2.07 months 4.48 years 1.16 centuries 3.03 millennia 78.7 millennia 2,046 millennia |
Source (and the entire article, recommended reading)
http://onemansblog.com/2007/03/26/how-id-hack-your-weak-passwords/