Motherboard networking, being soldered into the motherboard would not be upgradeable in a sense, but they are in fact as upgradable as any PCI based card. You upgrade a MoBo network connection that same way you upgrade a PCI based one, you buy a new PCI networking card and either move or disable the old hardware.
Wondering what proprietary connection would their be for WiFi ? it's wireless ... with perhaps a standard antenna screw on connection
2. Powerline networking oft has interference issues and offers hazards related to power surges.
3. Built in WiFi also comes with external antennae.
4. I don't see PCI versus built in as having any inherent performance advantages... those being more related to the quality of the individual components. Is this $6 solution better than that found on a $400 MoBo ?
This is the best answer on this thread:
http://www.tomshardware.com/answers/id-2960513/buy-onboard-wifi-motherboard-wifi-card.html
and I guess it answers your query well!