I would say it has nothing to do with accuracy. It has more to do with the perception that an in-house movement is less mass-produced, more hand-crafted, and thus, somehow, instrinisically more desirable regardless of its accuracy as compared with an off-the-shelf ETA. At some point in time that was probably still true. And I can see how, for the collector, it would be important. I get it. But, today, I'm just not sure how much it applies because I think most mechanical movements today - whether in-house or OEMed - are the result highly mechanized and standardized industrial process, and have largely been so since the Industrial Age.