well, not always ... but there are things like Mahler's 10th which was largely complete in a short score/sketch form. In that case other composers/orchestrators have had their way with the contents and made full scores.

Then there's a situation like Sibelius' 8th which has been rumored to have been written, then burnt. But recently people have found 'sketches' of the score ... and you know there are going to be some composers who may decide or be asked (by the Finnish government or some orchestra) to finish up on what possibly exists. I think there was even one person who 'wrote' what he imagined Sibelius' 8th symphony would've sounded like. ' />2' /> ... what the hell?

The most ridiculous 'unfinished' work is The Planets by Holst. You know, it's a 7 movement suite (Mars, Venus, Mercury, Jupiter, Saturn, Uranus, Neptune). In 1918 when it was completed, Pluto wasn't discovered. Holst died in 1934, 4 years after Pluto's discovery, and since, some people (usually on commission) write a 'Pluto' movement to extend it ... the only one I've heard is horrible and doesn't fit in well at all with the other 7 movements. I mean, maybe its not horrible, but its not Holst either, and a number of CD recordings include this 'Pluto' movement. ' />

Mozart's Requiem is another, he didn't complete it, one of his students completed it ... nobody actually knows how much is Mozart's and how much was written by the student alone. (Well, I think it's hotly debated anyway amongst Mozart circles ... )

The Art of Fugue by Bach was left incomplete and people always look at that.