Late reply to this one, but hopefully it will help someone.
The short answer is that Rollback Rx has its own defrag feature within the program. Running a defrag program will create problems when you create new recovery points, so using the Rollback defrag is the better option.
There is discussion on this defrag topic in the forums on the Horizon DataSys web site, so I won't get into it more here. Hope that helps. =)