A single micro-price (e.g., volume weighted mid adjusted for recent trades) is simpler and can be used for pricing both our bid and our offer.
But a bid fair and an offer fair have the desirable property that they widen out automatically as the market goes from one tick wide to five ticks wide.
As an example, given a futures book of 20 @ 6.01 / 6.03 @ 50 we would have a micro-price of 6.016 (since the bid has less volume) or a bid & offer fair of 6.012 & 6.026 (again, the bid is weaker).
Which approach is better and why?
EDIT: The above example shows two choices, a single micro-price of 6.016 or a bid/offer pair of 6.012 / 6.026.
If the market widened to 10 @ 6.01 / 6.09 @ 10, the choices would be a single micro-price of 6.05 or a bid/offer pair of 6.01 / 6.09. That shows one of the main advantages of using a bid/offer pair, they widen automatically as the market you are pricing gets wider.