Hey Rich!
I didnīt race them yet so canīt tell you if they are really necessary. As far as i know Fredrik designed the Rear Cover first (the long one) which has the advantage that stones and debris canīt get to drivetrain easily.
When he designed the Front Diff cover i think he worked ove rear one again and shortened it. Advantage is that it is lighter bus not as effective to hold stones and dirt outside. I hope this is at least a bit correct. If not, please correct me, Fredrik
All i can say that the quality is perfect and from what i heard from people who raced it, the covers really do their job, are light and very useful. You donīt have to clean your diff as often as without the covers and it will last longer.