Scenario:
You have internal users (employees) and external users (customers).
For internal users you want to use LDAP auth, but for external users you want to use MD5 method.
If a new field is added on users table (AUTH_METHOD DEFAULT NULL) it would be possible do this:

Get user from Mantis DB ( you always do this)
if AUTH_METHOD is NULL -> use method configured on std config parameter
else
use authentication method, defined for this user.

May be you think is a silly feature, but can be useful may be if you want to manage not only the scenario described before, but if you want to swicth gradually your authentication method.

I will try to write the code and send the patches.