$ uname -sr                                                                     
FreeBSD 8.2-STABLE
 
$ sudo -V       
Sudo version 1.8.1p2
Sudoers policy plugin version 1.8.1p2
Sudoers file grammar version 40
Sudoers I/O plugin version 1.8.1p2


According to "man sudoers":

use_loginclass  If set, sudo will apply the defaults specified for the
                       target user's login class if one exists.



But the test shows that the login class isn't changed after sudo:
 

$ grep use_loginclass /usr/local/etc/sudoers
Defaults use_loginclass


$ cat /tmp/showclass.sh 
#! /bin/sh
pw usershow $(whoami) -P | sed -n "s|.*\(Class: .*\)|\1|p"
 
$ /tmp/showclass.sh 
Class: 
 
$ limits -Sn
Resource limits (current):
  openfiles-cur            2048

$ sudo -u user2 /tmp/showclass.sh 
Class:
 
$ sudo -u user2 limits -Sn
Resource limits (current):
  openfiles               2048                                                   


However, after "su -" the login class is applied: 

$ su - user2
 
$ /tmp/showclass.sh
Class: user2class
 
$ limits -Sn
Resource limits (current):
  openfiles               10000


I looked at the sudo source code and found out that the "def_use_loginclass" variable isn't used at all. Instead, login class of SUDO_USER is always used.

The attached patch fixes it.
After the patch, sudo will honor the "use_loginclass" option and use target user's login class:

$ sudo -u user2 /tmp/showclass.sh 
Class: user2class
 
$ sudo -u user2 limits -Sn
Resource limits (current):
  openfiles               10000