It's not expensive, it's inconvenient. Being able to quickly jack in to a gigabit network and not have to worry about fiddling about with wifi keys and unreliable speeds is something you should get with a high end laptop like an MBP.
I've been using (and carting) a Thunderbolt to Ethernet adapter since I bought my first Retina in 2012 and it's really not all that bad. If you're constantly using it daily then carrying it around isn't a huge ask.
