I not sure if I will just open up a can of worms with this but here goes.
Right now it doesn't appear that my state Qld are enforcing countershaft sprocket covers, but they have always been required on all bikes under Pre 78 because the front sprocket is more than 30mm from the swing arm pivot as per GCR 12.8.8.5.
Please note 12.8.8.1 is referring to primary drive (eg Maico) that is why (the drive connecting engine to clutch) has been added.
12.8.8.6 is referring to the rear sprocket it really should say (chain guide) all modern bikes have a chain guide that can prevent something being drawn into the rear sprocket. I believe the reason it says (other than CMX/CDT) is because older bikes don't have this.
That means all bikes with the front sprocket rear teeth within 30mm of the swing arm pivot do not have to have a sprocket cover (from about 1980 up) when racing in modern MX meetings.
However in last years MOMS 2012 the rules 18.5.1.9, 18.5.2.8, 18.5.4.8, 18.5.5.7, 18.5.6.8, 18.5.7.6 and 18.5.8.6 were added which state "Countershaft sprocket covers will be fitted".
So if you haven't got one maybe you should get one. Also bar pads are required on EVO, Pre 85 and Pre 90 bikes, but for some stupid reason not on anything older.