I tend to use the grapnel when in shallow water (2-5mtr)so see no need for miles of chain; it will only end up getting snagged. My boat is around 2 tons and my 10kgs grapnel holds it just fine.
The MCA guidelines for coding for a 6 metre vessel are: "10 metres of chain or 20% of the minimum required cable length (whichever is the greater) of chain between the anchor and the rope".
These are general guide lines and not specifically for ribs. Compared to a lot of boats, ribs have almost no air drag (freeboard+cabin) and will tend not to yaw as badly as a yacht and for that matter donít have a mast and rigging increasing windage so 10mtrs of chain is probably over the top.
I suppose that if you were wreck diving or deep sea fishing miles of chain would be useful but around the UK coast Iím not sure, you can carry too much stuff. Having said that I have been experimenting with sea anchors and found that 1 ton dumpy bags are really good