Tracing paper was used by Stohlman for one reason, he did not own a photo copier. If the books being sold today reflected modern technology, tracing paper would likely stop being taught. Similar to a secretary training book written in 1963 would tell you to use carbon paper if you want a copy.
The modern day method is to dampen your leather, wrap it with seran wrap, lay a photo copy on top, trace it with a red pen (so you an see what you have missed), you are done.
Every time you copy your pattern by hand you are adding to the error factor. It is also a lot slower to hand draw a copy than it is to hit print on a copy machine. Also a stylus does not show where you have traced and missed, a colored pen shows up.