I've seen a bad side of this using eDonkey (which does this) - sometimes you'll wind up with a bunch of people missing the same parts of the file and no one having the full thing. This usually happens when the original source vanishes before anyone else gets a complete copy.

It can be very frustrating since you never know if you'll ever find the other parts or not. I've spent days trying to download large files only to find it was all a waste. I'd rather know beforehand that the full file exists intact. A popular file will still become well distributed over time.