That surprises me somewhat. Even on stages like Hammerhead, there are pretty important alternate routes and you're not always going to be on grates even if you take the most direct path. I'd imagine that, if you're already carrying at least 3 Mains of Run Speed, Ink Resist pays out much better than more Run.
Off-topic but I'm curious - what's wrong with those two weapon/ability sets? Does the Luna want to strafe slower to get more consistent directs and the ZiMi just not want to spend more slots after its usual Run Speed investment?
Movement speed doesn't really start significantly scaling until after 4 mains, so if we're talking about stacking we're talking about ~4 mains. Ink resist doesn't do enough to let you just walk through enemy ink anyway, so if you're stacking run to sprint on the grates and don't otherwise need ink resist I don't recommend using it.
Swim speed on the other hand I can definitely see ink resist being a good idea since you're going to be moving through ink while swimming.
Remember that if we're stacking (not just adding a little bit of extra movement, seriously stacking), we're building for that one game-winning push. The reason to stack run speed is for the grate sprint, and ink resist isn't going to help you there.
Run speed doesn't affect blasters while firing so it's kinda wasted slots for combat, and ~4 mains swim doesn't really allow for zimi to stack the run it typically wants (although sanic zimi is definitely viable). Swim speed zimi is at the point where you're better off with tri slosher or something because zimi is so gear-dependant, where as luna really isn't gear-dependant so it can afford to build seemingly worthless abilities for the rainmaker.