Allow for increased security with an option to turn on high entropy passwords (more than the current limit of 20 characters) and/or 2 factor authentication. It's not like longer passwords will fill up the space in your data center:

https://stormpath.com/blog/5-myths-password-security/

There's also this infamous comic from XKCD:

http://security.stackexchange.com/questions/6095/xkcd-936-short-complex-password-or-long-dictionary-passphrase