What happens if you just don't want an adaptor? There are only 2 USB ports on Macs and there are times when I'm using both USB ports and the thunderbolt port for an external display. I don't sit at the same desk every day so I can't get a docking station to provide ethernet and I don't always have wireless.
Not everybody's needs are the same and I know a lot of techies that simply wouldn't buy a laptop without an RJ45 port. Stuck at the arse end of some broken equipment at 2 in the morning trust me you just want to be able to plug the cable in and get on with it. And if your adaptor failed for whatever reason you'd look a fool in front of the customer because you can't plug in an patch cable on your £2k laptop!
When I worked in an environment like that I carried an extra long ethernet cord, 2 thumb drives with identical data and 2 power adapters with my single laptop. So the laptop became the bottleneck and if it failed I was out of luck. But, since I brought 2 more copies of my presentation or data, I could easily grab an unfamiliar laptop (borrowed) for the time I needed it. Not convenient, but my laptop failed far less often than my accessories. Had I been in your shoes where at 2am I could be stuck, I'd have my extra long cord because the ones provided in offices are often relatively short in comparison, 2 Thunderbolt to RJ45 adapters and 2 MagSafe chargers (they fail often as well). Without power or connectivity I'd be dead in the water so my bag would require a tiny bit more space for one more ethernet adapter.
For users such as yourself, your computer bag is probably both heavy and full. No sense ever leaving home without being fully armed for critical work. That extra adapter as a backup "just in case" item would be invaluable for the situation you described. Yes, a pain and extra expense, but for the real world it's an easy investment decision.
2 Thunderbolt ports, 2 USB 3.0 ports and HDMI port on a retina Macbook Pro. USB 3.0 hub would make the port issue a non issue. Yep, more equipment, but if you're hooking to everything you need in an office without a docking station, the hub would be another "fill your computer bag" item and you may also need a backup if it fails.