Defrags can sometimes cause the system to become unstable, I personally don't perform them any more as I have had way too many harddrives completely die on me after a defrag(one 2 yr old hd, and two 5 yr old hds), and other times just needing to reinstall windows after a defrag. For a system tool that is supposed to optimise your system, I can't exactly say I ever noticed any real benefits in terms of speed, and if so they were very minor.
I would advise caution when using this tool, I used to be an advocate for it until my last hd died after rebooting after a defrag. Also I'd like to add that I never really understood the reasons for the hds crashing, so it may have just been complete coincedence that it happened after a defrag.