The regular modules fit in pretty snugly and don't add any extra bulk to the device. So you can put it in tight bags or sleeves without having to carry anything else around while still getting whatever ports you need.

This ethernet module is a little bulky, but it still looks like it fits more snugly than any third party dongle would, and chances are you wouldn't want to take it with you when you leave the house anyway.

The bottom of the expansion cards is exposed though - it seems like it should be possible to do the fold open style of connector that a lot of laptops have built in without it needing to expand past the edge. But maybe including the circuitboard as well took up too much space.