Hi harveyIt was pretty common back in the day, which is why I would never consider it worthwhile to remove the head specifically to make any alterations for unleaded use, the chances are the head would have to come off anyway sometime, and you might as well wait until then if you do want to do anything.
Seats falling out was common as well, regardless of whether the head had been skimmed or not.
When you say common, did it affect all 4cyl cars? What did Rover do about it? You never hear the problem mentioned in writeup's about the cars from the sixties and seventy's. Or is it a case of all cars from this period had their problems?
If a valve burns out you'll get a misfire, and if a seat falls out then you'll get a misfire and a nasty knock.What are the symptoms if this happens?
Would the latter wreck the head Harvey ?
There isn't any problem skimming the head. The burnt out valve looks a bit strange, as though it's got very hot and melted quickly, rather than over a period of time. The valves look to be very recessed in the seats as well, and that may account for the lip around the other valve. (But that maybe just how it looks in the photo).
I think a lot of problems with this is down to either poor maintenance or bad carb settings.