Why subscribe to STMFC if you didn't want build accurate prototype cars? Since the emphasis of this group is standard gauge of a relatively modern era, interchange is a fact of life so even with a freelance railroad it's nice to have the visiting cars right.
Of course if one is a secret masochist it might be another reason ;) I like earlier periods (pre-1900's), but even then there are useful tips that popup on this group that I can use or details like servicing journals that are fun to read.
Certainly if you need a large, varied freight car fleet, you may need some stand ins that aren't the best for operations purposes until you can get or make better cars, I always liked the NEBW grading system for cars, maybe we should carry it one step further:
A - Car is perfectly prototypical, no visible mistakes, perfect dimensions, an accurate color scheme and lettered correctly.
B - Car is close prototypical, no 'easily' visible mistakes, dimensional errors that require measuring tools to find, an accurate color scheme and lettered mostly correctly with maybe minor errors.
C - Car is reasonably prototypical, some mistakes that are apparent to knowledgeable people, dimensions may be a little off, an accurate color scheme and and any blatant lettering mistakes easily corrected with redecaling the car.
D - The car has a prototype of sorts, even if it is not the one it's lettered for and has about the right shape. Also blatant mistakes like wrong roof type or ends, colors that aren't that close to the prototype portrayed and serious reporting errors or other details that entail some serious paint or decal work to fix.
F - This is a fantasy car with no prototype, details are totally bogus, colors are way off and the lettering is mostly nonsense.
I assume all cars roll on the right track and couple, otherwise they don't count as cars and are 'scenic accessories'.
From this list you can see what I think is important when you look at a freight car in a layout context.
#1 The colors about right, no pinkish or purplish cars please unless that's the prototype's color! ;)
#2 The reporting marks look right, there are no blatant errors and the minor errors are not readily visible.
#3 Close to the right dimensions with no blatant error in details.
Your mileage may vary!