It makes sense only if you have a mixture of very slow and very fast clients.
For test only I have played with it and it does well what is suppose to do.
What I did: a far away client and a close client, both in 5GHz and both doing how much traffic as speed allows. With Air Fairness the fast client is doing 15-20% more speed - it got from ~250mbps to almost 300mbps. While the slow client kept its around 50mbps.
But that's a narrow case and it's not common to try to do lots of traffic with a client that has little signal (in my case it was -67db).
Main purpose of this feature is in case you have very low speed clients (802.11b) and fast clients (802.11n) or 802.11a and 802.11ac.
Fast client will be limited considerably by the slow client because slow client keeps the channel busy longer.
