If you've ever been to a sound stage and listened to 100 musicians playing music, you know that there is nothing else in the world like that sound. The reasons to take advantage of both the acoustic and the synthetic score are numerous. Another use is the blending of traditional sounds for supplemental purposes. First, let's discuss timing. Ideally your sources of click track for pre-records and for the live performance are one and the same.
Metronomes vary substantially, and in music for picture, the tempi can be quite complex.
To add complexity to this procedure, some samples tend to speak late, so the necessity of having one timing source that you know all the various tracks will reference to is of paramount importance. If you are accustomed to having a music editor provide clicks in the studio, consult with him to find your best approach for pre-records. If he's built clicks in the Auricle system for your session, you can take advantage of the extreme accuracy of that system in your pre-record session by externally slaving your sequencer to the Auricle using midi beat clocks. Always record click to a track of your pre-record tape for safety, and if your click source is consistent throughout the process you can safely have the musicians play either to live or to recorded click. If using the click on tape, it still is best to have a source of live click in the studio for rehearsals and pickups.