The problem with rack and panniers is that they make the bike awkward if you have to lift or carry it. There were only two locked gates on the route at the weekend and very little hike-a-bike so it wouldn't have been a big deal but in general for this type of event everyone uses a combination of the handlebar/frame/seat bags. I've rack and panniers for road touring. Having smaller bags encourages you to think about what to take.

With my frame bag and frame I can't fit a water cage in there but again for this event it didn't matter as I was using a camelbak. I went for the Camelbak as I knew there were big distances and time between replenishing points especially since we'd had dry weather for a while and there'd be nothing on the tops. Some frames have the bottle mounts set lower so it might not be a problem, also most of the framebag manufacturers do a custom option - you send in the outline of your frame on a large piece of card so it can be made to wrap around water bottles or even shocks if you are riding a full suspension bike.