I'm pretty sure that it's just planning for the most likely scenario, rather than the worst-case scenario.
Without darts, the staff-wielding magic user would have to choose between approaching the monster, or doing nothing at all. I'd rather be out of luck on the rare occasion where I'm forced into melee, than be entirely helpless in every situation where I'm not forced into melee.
It really does seem like the dagger is the right choice, though. Did they weigh much more than darts? That could have made the idea of dagger-throwing less ideal. It would still be preferable to a staff, of course.