When is a ethernet-to-thunderbolt cable necessary? With the upgraded wireless on the latest 2013 MBP's when would I need this cable? I do have a Time Machine but the rate of data transfer wouldn't increase from having thunderbolt since the speed would only be as fast as its the slowest technology. 

What else can I use it for?
The main reason for Apple to develop the Ethernet-to-thunderbolt cable is because the Macbook Pro retina line is too thin to fit the ethernet Port, thunderbolt is thin enough to fit. If you have a MacBook Pro retina you have 2 choses:


Use Wifi
Buy the Ethernet-to-thunderbolt cable to use cabled internet on you Macbook Pro retina


The only reason: People who don't have wifi, don't want to use it, don't have a fast rooter,... 

If you have a normal unibody macbook pro there is no reason to buy the cable. :)