To me, most weathering that I have seen for steam locomotives is unrealistic... i.e. rust streaks where there was no water or steam source, sand where there could not possibly be any, way too much color variation on the model, etc. Those who do this work realistically appear to do the necessary research that includes knowing the basics of a steam locomotive and how it operates, all the sources of water, steam, and exhaust and looking at photos, etc. Some painters paint main and side rods. I have never seen or photographed painted main or side rods in service either on NYC or NKP. The rods were a yellowish brown, probably as the result of exhaust steam from worn piston packing mixed with valve oil.
While I recognize the existence of old and weathered rolling stock, I prefer to think of the railroad as a "better maintained" environment than it actually is.....
I would not buy a weathered model at any price, since a lot of "artists" who weather motive power always seem to overdue. However, some modelers like weathered models, so beauty is certainly in the eye of the beholder.
Some years ago a magazine photographer photographed my layout for a model railroad publication. He asked me to position engines and trains in certain areas of my layout, and was genuinely surprised that my layout actually "ran". He told me that most of the layouts that he photographed were more like dioramas, that fewer than 3 out of every 10 could actually be operated, and that their owners were more like "artisans". I concluded that I must be in a different hobby than many of those whose layouts appeared in print.