No question, the sound operator needs to be on comms. They may need to get cues, be aware of any problems or changes, know when to unmute inputs when somebody is about to go on stage,
etc. If you have a lot of beltpacks I find it difficult to believe there isn't somewhere else that you could better go without.
If nothing else, the sound tech is often the one responsible for the comms system, you might want them on comms in case somebody has a problem with it.