Yes--others have noted that too.
Ways to reduce/eliminate corrupt downloads are already being submitted and developed, so watch for the beta releases. (Search here for "THEX"

Incomplete folders are not shared/ignored, but yes, some users put incomplete files in their shared folder, or rename the incomplete folder.

Too, MediaSentry admitted in court that they distribute bogus or inoperative music files (see page 11 here. Maybe someone is doing the same with video?