The main point of the self contained expansions is that you are unlikely to loose them and they can safely travel around with the device. With a card that protrudes like that, if you want to put the laptop in a bag, you have to take the card out, or risk damaging the card and/or laptop as the protrusion gets levered around. The expansion cards plug into USB-C connectors that are soldered directly onto the motherboard, so the risk of damaging the motherboard though the expansion card does exist (although it's probably not that likely, and any damage would probably be localized to just the motherboard's USB connector).
With a external Ethernet port, you can have the USB expansion card in between the motherboard and the USB Ethernet device, to prevent wear on the motherboard's connector when repeatedly connecting and disconnecting the device.
So what's the gain by using the expansion card format, when it can't permanently live in the laptop? You might as well just use a cable. I guess maybe if you only carry the laptop by hand and have to constantly use Ethernet, like if you're debugging a network installation, it would be good? But a regular USB Ethernet port seems like it would be more practical for most cases.