This is a long running discussion, with vocal parties on both sides. Let's
remember, SharePoint is designed to be an end user driven product. That
means, it is meant to be managed by the users, for the users. Site
Collection admins and Site Owners have the ability to add users to their
sites, and should be adding users to their sites. If an AD group exists,
that fits the need, then sure, use it. If not, create a SharePoint group and
add the proper users to that group. There i8s no need to get AD involved.
There is a very good section in the Best Practices book about this. Trying
to manage SharePoint access permissions via your AD is a mountain you really
don't want to climb.