I see your point, I agree it's generally best, especially in a case like this where it doesn't add much.
I think you actually answered my question with:
1. This explanation that this is not obligatory and that it's defensive.
2. Mentioning that `if there is no reason to believe that the directory would vanish between the evaluation of the for loop's first line and when you need to go into it, the if statement may be removed`.
3. With the example to a non defensive alternative.
4. With explaining why it's important anyway.