That's not true at all... Spell Interrupt Rate Down does not have anything to do with your chance to interrupt a spell while moving. Moving in FFXI always prevents your spell from going off. The spell will continue to cast until it reaches its completed amount, then it will stop and do nothing. The ONLY way to move, and still complete the spell is if you start casting it, run, then run back to the exact spot you began casting from before the spell goes off. So if you run in circles while trying to cast a spell, theres a small chance you will not cancel it because you may still be on the same spot when it does go off. If you don't believe me, go try it. Find a rock on the ground to mark your spot, start casting something with a long cast. Run around for 2 seconds, then go stand back on that same rock. The spell will always finish, if you make it back in time. Otherwise, it will always cancel.
http://ffxi.gamerescape.com/wiki/Spe...erruption_Rate
The third bullet point on that page explains it. There is also a lot more information on other sites about it.
I really wish they'd keep the ability to move while casting in this game. Can you imagine how annoying stuff like darkhold is going to be when they make it interrupt when you move? you'll have to stop running through the mobs every time you need a heal from yourself or the healers. And there's really know way to beat a speed run if you plan to kill the mobs through the dungeon, so running through is always the only strategy for it... So you can't just say "well dont run through them, and fight then." Since speed runs are required to get certain drops, as they only drop if you beat it in a certain time frame.