Say you have REASON/RECORD and Cubase sx, why would you rewire to Cubase if you have RECORD? Is it due to the fact that you would beable to use other VST in CUBASE? such as kontakt, battery, reaktor as well as reason and record?

Yes ! Plus you can apply VST effects from Cubase to your Rewire channels coming from Reason via Rewire, and Have the option of mixing/editing/exporting the entire project from Cubase.