I think if you have an event organiser supplied by the venue, they would do more or less the same thing as an on-the-day coordinator.
With that plus the toastmaster, I think you'd be getting into 'too many cooks' territory if you brought in your own coordinator too.
The owner of our venue acts as the coordinator for the day, as well as making the announcements throughout the day (pretty much toastmaster). Our wedding is Sept this year, and we met with him on Saturday to go through our suppliers - he is going to liaise with them regarding set up, equipment and pre-wedding visits.
If you feel that the people provided by your venue (particularly the organiser, the toastmaster doesn't really come into it until the day) aren't doing what they should be, I would contact your venue manager. You are paying for their services, so if you feel that you need to bring in external help, they really need to up their game!
Good luck, hope this helps! X