Because marines should have more presence. We're meant to be rescuing people, not having PIRATES rescue people. It's not as bad as hearing that the only marine someone knows is a corrupt psychopath but I'm far from the only marine on the Grand Line. Makes it seem like we're outnumbered even when we aren't or worse; that the others don't do anything.

You could probably ask Roronoa more about her. You mentioned liking the faces he made when Lucci bit him, bringing up Tashigi is likely to have a similar effect.