A staff deals magic damage on hit and benefits from the skills I listed previously.
Not all warfare skills deal physical damage.
Once armor is down then all damage of that type is equal. So, if I have a bunch of mages and some meleers with no armor because their armor got shredded earlier it actually is more efficient for me to use Crippling Strike on my Mage than to use spells since I can get kills.
Also, my mage isn't exactly losing any specialization by dumping points into pyrokinetics and favors wielding a staff.
You asked, thematically, what the point of a staff is and I am telling you that it provides non-trivial damage.
