Well thats not possible, they both have the same damage modifiers, so either you haven't thoroughly tested the two, or you just used kasha once and saw 200damage and never used it again. Or maybe you were fighting Jnuns, in which case, my gekkos do over 2k damage with 300% tp on.
Light will always be better than darkness. Especially during HNM's. Thunder is the strongest magic BLM's have, therefore Thunder IV MB off light works wonders, darkness cannot do that.
Only time you should actually use the WS's for their added status effect is if a mage type mob is casting a spell(gekko) or if you're soloing and could use the extra paralyze on the mob, or even blind. Otherwise its pointless, considering they don't last very long, and as you said, thats the RDM's job.