My mother (Emergency Medicine MD) seems to semi-routinely get caught in these types of situations. Especially since the other MD(s) on board seem to all be radiologists or pathologists.
As recently as 10 years ago, she and my sister (travel companion) were upgraded to 1st class for the rest of their trip and the return on a flight to Egypt.
On NW, she recieved 5,000 miles and a thank you.
Your mileage may vary -- but in some cases there is no other option.
Also -- even though they have doctor's on call (via airphone), flight attendents cannot start IV's or give any drugs. So esp. in an emergency, having a doctor (or an RN who can take the on call doctor's orders) on hand becomes vital.