Its not that the clutch wears so much that the flywheel needs replacing, its that the flywheel on some is Dual Mass, and this should be replaced with the clutch, as the mechanism inside will wear, and this give symptoms like a slipping clutch, but if left, the clutch can be damaged too, thus both needing replacing again.
I recently had to replace the clutch on a Vectra C as the DMF had failed. The clutch had only been changed 10 days earlier. The owner thought that they could save some money on not getting the DMF replaced as well. In the end, they had to replace the clutch (Twice) and the flywheel, as well as pay for the labor to strip it all down twice.
They insisted that they didn't want it replacing the first time. I remember as they drove it away after just the clutch "Well, The customer is always right..."