It doesn't fit. The RJ-45 connector nearly the same size (possibly larger) than the edge of the retina MacBook Pro.

Possibly, but I've used only an Ethernet port on my laptop three times in the last year and I don't think I'm that far out of the norm.

Once 802.11ac comes out Ethernet might finally be relegated to "the cable that I use to hook up my base-station to my ISP's router."