This would work well for calculating the number of cars necessary to make a general layout.  But let us say you model a specific area where one industry is a staple item?  A terminal with an Ore Dock will have a much different freight manifest inbound versus a general town. And then there is the difference between a steel mill city and a textile mill city - while some general statements will stay true, others may shift.  Your balance will be shifted towards the car types used by that large industry and some others might be nonexistant altogether.

In this case it might help to get a roster of the yards on any given day in the time you are modeling and calculate percentages of each car type from this list or by visually counting each general car type [Yikes!!  If you think that sounds scary, don't become a geologist!! ;)], and then use the general porportions to calculate the specific car types.

Just some food for thought.