I've posted about this many times....
There is a private campground near the east entrance.
The Pros (as I see them):
It is literally one mile from where the trail ends. I have clocked the distance in my car. I believe they also have showers there. It's not nearly as nice as being down in the canyon in my opinion, but it's likely quieter. If we stayed there, no shuttles at the end would be necessary. Just add an extra mile of road and you're at camp. Nobody would have to wait around at the end- people could just trickle in.
If anyone bailed at the Grotto and it was late, I'm not sure how you could get back to camp. The Grotto is down in the valley and there is no sort of public transportation to get back up to the east rim. Someone would have to leave a bailout car down in the valley. How people would contact each other if bailing out or needing a ride down there is beyond me.
Camping in the valley:
Pros: In my opinion, its better camping; on the Virgin River. If people bailed at the Grotto, they could either walk about three miles from there (all slightly downhill road) to camp or possibly catch a shuttle if they're running. There's plenty of traffic back to camp.
You'd be finishing pretty far from where we camp, basically needing shuttle at the beginning and at the end. There is no public transportation to get from the east rim back into the valley and you can't walk it (no peds in tunnel, the only way back down)...We'd have to leave cars/have people wait at the east rim for everyone to come in. There's parking and picnic tables there, not a bad place to wait. But if someone bailed down in the Grotto, I don't know how the people waiting for them at the end would get word of it....
You really need to look at a park and road map to understand this.