Re: back up dinghy?
I can see the utility of having two dinghies as described above.. add a few kids into the mix and it makes more sense still. If you have the space to keep the stowed roll-up why not? Eventually a small kicker for it will be even better, and it can be a backup to the RIBs power. If you're off on an errand with the RIB, anyone left behind isn't necessarily boat (or beach) bound while you're away. Swimming may not always be an option.
We carry a 7.5' rollup (with a 3.5 o/b) essentially as a 'backup' to our two kayaks. The dinghy comes out of the bag mostly only if we have guests.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"