Often it is. Earlier in this thread I used the example of a unit system - one example of there there can be a ton of repetition to remove, but there are fundamental differences between liters and meters that make removing the duplication hard if you didn't realize upfront you had a problem. Once you get it right converting meters to chains isn't that hard (I wonder how many reading even know chain was a unit of measure - much less have any idea how big it is), but there are a ton of choices to make it work.