Under Part 91, not for hire, the visibility is our official minimum and the rule says that flight visibility is all that counts. So if we are compelled to consider that the weather can be worse on the final approach course than is reported at the airport, maybe it can also be better. We are free to fly an approach regardless of reported weather to see if indeed the flight visibility is such that we can see the runway at the appropriate time. We are also free to crash, as many pilots have done, trying to make it into an airport with a reported quarter of a mile visibility and a vertical visibility of 100 feet. To land, all you have to do to be legal is swear that the flight visibility was equal to the published minimums and that you saw the runway (or other applicable things) at the proper time.