We had lenghy and heated discussions about that very topic nearly 4 years ago now when designing our 1st SR-IOV drivers.
There is no technical reason why you cannot have a single monolithic driver. We decided to separate the drivers for simplicity sake. The VF drivers change much less frequently than PF drivers, as we are always adding new features and fixing issues in the PF driver.
With the separate apprach, the PF can be updated without affecting the VF, and therefore the VM.
There was and still is no standard no any issues with either approach.
Hope this answers your questions.