Not always. Most professional bands will have a core group that plays at the majority of weddings they are booked for. However, they will also have a group of deputies (or “deps” as they are known in the industry) that they call upon frequently to cover for band members. This is a standard and essential practice in the music industry. Most musicians work as freelancers, meaning they might work for different function bands or tour with famous pop groups for example. Being able to draw upon a deputy musician to cover a band member who is unavailable (or ill) means the rest of the band can still work.