If we consider a preset, and I've said some more on presets here, the sound designer has no idea of the tune you are going to use it in, save for some idea of perhaps the range of notes (e.g. for a bass he/she will not expect you to be playing a tune two octaves above middle C), or the genre (house, trance, techno). Now:
- When you are composing and arranging a piece, before you have finalised what melodies, chords, and such that you are going to use, you only want presets to be roughly what you want.
- When you have finished composing and arranging, then you know essentially exactly what synths will be playing what notes when. Once you know this, you can design a patch specifically for the part it plays in your track. You have knowledge a sound-designer authoring a preset does not have: you know the track you are writing and he/she does not. Make use of this extra knowledge.