At least with version 1.235 of Hudson it is impossible to assign users to groups
when using Hudson's own user database and Matrix-based security.

Adding a proper group management would benefit users at least in case when one
would want to set up Hudson in a non-complex way (without LDAP integration and
such) and still be able to use role-based differentation. For example, the
default anonymous users (corporation management and marketing) could have read
only access, Admin users (IT infrastructure management) could do anything, and
Builders (Dev team QA) could create and execute jobs.