Hello Vikram,
A sunken rib is sometimes a single piece, needing no bottom rib, so there is no hidden part of the barrels between two ribs. In addition to the weight reduction where it counts most, over the weak hand and up forward, there is no chance of rust starting its destructive work in a hidden and inaccessible place.
Take a look at the current thread on blown up barrels to see how much corrosion builds up between barrels.
In my experience sunken ribs are more frequently seen on French doubles. The French have also developed an "I" beam section on the rib of the Ideal. It sort of slides between the barrels and there is no need for the clamps and wedges rigmarole used to fix conventional double ribs.
In case there is any doubt: I detest ribs because I have seen what rust can do between the barrels.
