I just finished designing a bicycle shaft. I didn't do a very thorough job.
It is not because I don't know how to do it, but that fact did play a contributing role. It's more due to the fact that I can't figure out how to graph functions in MathCAD. Plus, a drawing that would take fifteen seconds on paper takes fifteen minutes in Paint.
But paper and pencil have drawbacks too. I get tired of punching numbers into my calculator, and also it's easier to erase and format your page when it's digital.