Anyway, I work with sound libraries. Well, nowadays everyone does. Even hollywood soundtrack composers, they only record with orchestra on the final stage. What I do is playing the notes on a midi keyboard or writing it directly on the computer, then assign one instrument to the line I've just written and there it goes, there's a violin sounding. Sound libraries nowadays are huge, you have every note of every instrument and every articulation (staccato, pizzicato, etc.) recorded with quality. Problem is when you record your line played on piano you have to edit every note to give it the character of something played on a real string instrument, brass, etc. which you can't do on piano. At the end you spend much more time editing than actually recording music. But well, I guess that happens in any kind of musical recording.