I recently met with a client who will be working with us for wedding day coordination. She began her planning with the thought that she wouldn't need her own wedding planner, since both her church and her venue provided on-site "wedding coordinators". In fact, the venue's coordinator, in her estimation, was "meticulous" and "reassuring".
The problem? In the year since she booked her church and venue, both coordinators have since left for other jobs, and she is now dealing with two totally new people; having to explain all of her wishes and instructions all over again.
I have found this to, unfortunately, be pretty common. In the corporate hospitality industry, job turnover is rampant. The nice thing about having your own coordinator that works for you and not a venue, is that there is little likelihood of job turnover, so your plans stay with the same person or consultant group throughout. The coordinator is loyal to the client, instead of to the corporate ladder. In 11 years, I have thankfully never missed a wedding (knock on wood!), and I know that my clients have been reassured by that.