I’ve never needed more than Roomworks, and, if you haven’t already upgraded to C7.5, REVelation is Roomworks on steroids. I can’t explain why people need more, but hey, it’s always easier to buy another plugin than to blame anything else. When I had only Rookworks, SIR was a welcome addition, but since REVerance added convolution reverb to the Cubase arsenal, I’ve been sorted.