Agree that the oem oil's are overpriced and may not be anything special, and for the most part I don't use them either. BUT, say your on a new engine development team and have ran all the hours of durability testing. Some of which is pretty extreme, like 300hrs wide open full load. And you did this testing with oem brand oil. Would it be out of line to recommend it?
Or if during the proving it was found that the engine couldn't pass a certain test. Working with an lubricant company you find a solution that is cheaper than a parts redesign. Then you recommend it to the owners. I know this has happened in at least case.
AGAIN, I'm not saying other oils are not as good, just something to think about. A friend that works for FMC says in some applications anything but oem is an "experiment".