Ted,
I am forced to use a conventionally ribbed double because there is no real alternative.
Like you say, a Darne with no bottom rib offers no place for rust to form. Some English guns, not all, had the space between the ribs tinned, but even then rust could form in some untinned parts, and I am talking from experience here after having seen many guns undergo rib relaying.
It is perplexing to see makers go to the latest high tech machines and still maintain tinware technology for the ribs.
Admittedly there is an aestehtic factor at play. Ribless doubles, SXS and OU, can look awkward without ribs. Well, that can be fixed without recourse to the ironmongery of ribs. As for the pointing aid, (for game guns, not target work) the eye can be fooled into perceiving a whole rib even when there is only a short rib over the chambers and a short bridge at the muzzle, that is how the first Boss OUs did it, and it worked.
