If you are like me, and find it frustrating that you can only connect two Thunderbolt devices (unless they are daisy-chained) to your Macbook Pro, even on a 15″ one, you have two external screens, but you want gigabit ethernet, so you either have to sacrifice a Thunderbolt port and second display, or fork out for an expensive display that can be daisy-chained. Or alternatively, find a HDMI capable external display to use the HDMI port, or a Thunderbolt dock that has built in gigabit ethernet. All viable options, but none is really cost effective while maintaining dual external displays.
I personally think it is crazy that Apple have not made their USB ethernet adapter USB 3.0 and gigabit ethernet capable. I do understand that when there is a Thunderbolt one it may not be necessary for everyone, but for me. It’s nuts. I either have one screen, or I have 100mbit ethernet. Neither of these is ideal either.