I played in pit orchestras in high school and college, and it's surprising how much can change from show to show. Lots of songs have vamps (sets of four or eight bars you repeat over and over until something happens and it's time to go to the next bit), or holds, or other various other things where the conductor is paying attention to what's happening on stage to cue the musicians. You could probably rig something up in QLab or whatnot to maybe emulate that effect digitally, but it's never going to be "just hit play on the canned track."