- Lines with more than 66 non-whitespace characters decrease readability.
- Statements with more than 66 non-whitespace characters are common in most programming languages.
This means that you end up either with lots of continued statements or lots of overly-long lines in Python. If you have the former, then it's hard to see the indentation. If you have the latter, then you can see the indentation but the overall readability suffers. This can be fixed by using tabs for semantic indentation and spaces for alignment and an editor that supports highlighting tabs, but the Python style guides tell you not to do this.