Driving a truck, towing a trailer, or riding a motorcycle, and driving a prius are two very different scenarios.
Yes, with no center rearview, the mirrors need to be such that the operator may see what is directly behind - and this is when those spot mirrors are gold (bikes included).
With a car? Nope. Not required (but people have their 'preferences')
In 99% of all cagers, if the mirrors are tweaked properly, the driver may go from his peripheral vision on either side, to the side mirrors and to the rear view mirror and not miss a thing.
I guess I've learned to trust my instruments while flying.
No worries.