I haven't really found any situation where I did sekkax2. Even before I had kannagi I usually did WS DMG+10% uzura/kamome. If I needed evasion, eva sekka/kamome
As for mab Katanas on Nin I only bothered making one because well..
http://www.ffxiah.com/item/19272/yagentoshiro
While brewed it should be better than 2x sekka No? In most situations it seems to do better with flat 10% increase usually using MM Ultimate and beyond with /RDM
Great for low manning creamed corn (cerein croin?(sp) ), back tanking things like sisyphus and just a lot of fun in general. Done some 5k non brewed hyoton San to NMs on burst