I'm not sure if floods in and of themselves are entirely counterproductive, but I'm sure the Ascians don't want another fiasco like the Thirteenth to occur, causing potential damage to the whole upon rejoining. I suspect this is why Elidibus sent Ardbert and his group to the Source, probably as a way to hasten the First's rejoining before things went too far by having them begin the Eighth Umbral Calamity. After all, I get the impression that Black Rose was an improvised contingency plan afterwards due to the discovery of it's increased potency due to the change in the Source's aether brought about from the trickle effect from the First. I could be off-base though, as I'm still processing all the new info we have now.