Think of this real world scenario - a help desk guy accidentally 'nukes' a security group out of your AD environment. The security group in question had over 1000 items inside and had not been documented very well. Tapes from last night have already gone to off site storage and the idea of doing an authoritative restore just doesn't sound very appealing.
Not only can you stop and start Active Directory Domain Services in Windows 2008, there is also an AD Database Mounting Tool that can mount snapshots of Active Directory, assuming you have shadow copies available on your server.
If you want to read about the gory details (which isn't as bad as it might look) on restoring objects out of a shadow copy snapshot, read the excellent write up over
here.
The best part, you don't have to take the DC offline to do all this. You can manipulate/export the data out of the snapshot side-by-side while the DC stays online.