As described in "MODE 1", CMG provides an option to use a shared address book in GroupWise 8 environment. The shared address book option is provided, because GroupWise 8 does not support a Free/Busy URL within the GroupWise (System) Address Book.
A second architecture was developed at the request of customers asking for GroupWise 7 support, because GroupWise 7 does not support the Free/Busy URL attribute at all.
It sounds like GroupWise 2012 SP2 addresses some of the limitations of previous versions, which is great. In fact, these modifications were formally requested by the CMG team on June 2, 2011. However, it would be advisable to reserve comparisons until GroupWise 2012 SP2 releases and a version of CMG supporting the new free/busy access methodology is available.