David has also offered the following response to the same question: "Baker valve gear is primarily rejected because it has too many pin joints (more than Walschaerts, check for yourself). I suspect it is also heavier. Obviously opinion was divided even in America on the relative value of Walschaerts and Baker as some of the better railways (e.g. the Pennsylvania) preferred the former to the end. The only advantage of Baker that I know of is elimination of the sliding link/dieblock, and as you rightly surmise that problem is dealt with by mechanical lubrication as on the SAR 25 Class."