granting users to acls' as opposed users to groups

we have taken a review of a file server in relation to permissions. In most cases, access to a specific directory is based on an AD security group, full of members. But on a few directories, users have been added individually, e.g. their AD account as opposed to adding them to a group and the group to the ACL. I am aware this isnt particularly good practice, but need some feedback from those who admin file server permissions and AD as to why its such a bad idea. It is not an enormous AD or environment, approx 500 user accounts.