DB class is for database interaction. DBUtil class is for schema manipulation.
The ORM is primarily designed for interactive functionality, not for batch operations. It caches every ORM object to ensure consistency between multiple queries on the same object:
$userA = Model_User::find(1);
$userB= Model_User::find(1);
You will see that $userA === $userB, they are the same object, not two diffferent objects with the same content. So after you change a $userA property, $userB is also changed.
If you want to use the ORM in bulk operations, you need to disable the caching mechanism. You need the 1.9/develop version of the ORM code for that. With it, you can set the "caching" variable in the orm config file to false, to disable all object caching.