Incomplete files can be shared if done with some intelligence :

If a host is in the middle of downloading a file, it can report the original filename (without the "T-filesize-") and finished filesize to queries by clients.

If a client asks for a resume, and the resume location is available, the host will begin sending.

If a client should download all the way to the end of the incomplete file on the host, the host will keep sending it if the host can download it at close to the same rate it has been sending it at. If it is downloading it at a significantly slower speed, it will disconnect the client so that the client can look elsewhere.