Partial downlds help people to get faster downlds. Even though the file might be incomplete, what has been downlded can add to other people's downlds in a similar fashion to having extra sources to download from. And downlds will generally be faster & more reliable if you're downlding from more than one source.

As long as you haven't added your incomplete folder to your shared list of folders (very bad!) then allowing partial downlds is fine. Though you can also consider Only A Hobo's comment.