It's true that this feature is somewhat not easy to implement... But what made me post this thread was that Cubase and many other DAWs can do this. Maybe I should make a feature request...
As for why it is needed... For example, using Strings Staccato, there is an "attack" before the "rhythmic start point" of a note. If you simply set the sample start to that rhythmic start point, it would be unnatual. The only way to both keep the piano roll tidy and hear the notes in rhythm, is to apply a negative offset to the midi track.
Anyway, thank you!