I've always followed that train of thought too. In 20 years of servicing small engines, I've never once bought an OEM branded engine oil. And never once had a major engine failure.
OEM's don't have their own refineries and chemical plants. And they all follow industry established specs. I have no objective data to say they do in fact contract with the lowest bidder that meets spec. But it wouldn't surprise me. Oils can have a wide range of quality within the same specs though. And that's why I stick with non-OEM oils... They have to compete with each other and be scrutinized by users without the advantage of having an OEM label on them.
On the flip side of the coin, I'd have no concerns running OEM oils. Just personally can't justify the extra cost living on a budget.