Yes, the rollbar is a structural element in the vehicles strength and rigidity. The car was designed with the rollbar elements in mind. In fact, other manufacturers have followed suite (BMW Z roadster for one)
There is the chance that, should you roll the car, you could actively "duck" and have your head below the rollbar height.
Second, consider the impact of a re-sale of the car. It would take a rare buyer to want a car with the rollbar removed, or worse, removed and replaced with questionable welding. Perhaps a Spec Boxster buyer who would put in a cage wouldn't mind?
The fiberglass top will likely shred, dent or, even worse, collapse on your head. Having the rollhoops present may prevent it from impacting even more on your knoggin
