As you are aware by now the MacBook Air does not come with an ethernet port because of its slim design, so if you need to use a LAN cable for MacBook Air, this adapter is your best and only bet. Call me old fashion but I still prefer to transfer files over LAN as it is the speed is much faster and more stable.
This adapter is only capable of 10/100Mbps speed since theoretically USB 2 speed max out at 480Mbps which is much less than the speed of a gigabit network so if this were to be made for 10/100/1000Mbps, it wouldn’t be accurate and that is the reason why Apple supports 10/100Mbps only. Perhaps when USB 3 comes out, this will be refreshed to support 10/100/1000Mbps as USB 3 is capable of supporting up to 5,000Mbps.
I love how Apple design the cover for this, it holds the RJ45 port and the USB port side by side.