Eric, you tickle my ribs (pun intended) if you think a ribless roof won't leak just like a ribbed one
All 90/110/Defender roofs have seams in similar places, and Landrover was never especially good at assembling the component parts with sufficient sealant.
The ribs are spot welded to the top of the roof. The first 'ribless' versions do actually have ribs, they're just on the inside , and not as strong as the ones on the outside. This only really matters if you like standing on top of your Landy
The most recent (Td5 onward I think) roofs have minimal pressed in stiffening ribs, and I am unsure whether they are in fact of one-piece construction. Not sure I'd want to stand on one of these.
But probably the second most common source of water in a Landy is condensation. If you don't have a good headlining you will still get rained on each time you go round a corner, especially on a cold day when the ice on the inside is slowly thawing