If you're doing an MTI musical, OrchExtra as suggested above does work decently.
Notion (
NOTION Music - Notion version 4 - Redefinind Music Notation Software) is like OrchExtra, but let's you voice your own compositions, as opposed to a pre-made library. When I used it we had a
MIDI keyboard that let our operator tap-tempo for the entire Notion
orchestra. It is fairly common for low-tier broadway shows to go out with either a
QLab rig of recordings of the missing
orchestra parts, cued in tempo by a
conductor, or to go out with a Notion rig where the
conductor can conduct in tempo, and an operator on the Notion computer can tap tempo, vamp if there are issues on
stage, and leave the vamp like any good band member. It's not perfect, but it doesn't sound nearly as poor as poorly voiced
MIDI synthesizers, and if things happen on
stage it is better than any redundant Q-lab
system can be.