STR gives you literally no benefit as a (staff) mage. It gives you some more spirit points, but that's only helpful for spirit stance, which mages never use. The extra spirit as no benefit for spirit shield, which uses a percentage of base spirit. (Incidentally, the SP Tank ability adds a percentage to your spirit, which is actually helpful.)
If you share the persona with 1h mage, str helps with counterattacks and spirit stance/ofc, however you choose to spend your overcharge. Eventually, you may wish to buy a new persona dedicated to staff mage and leave str at 0.
I suppose I should post my mage stats too. 1h mage:

staff mage:
1h mage has a bit more wis because I can use the magic hit chance. Staff mage wis will go down relative to the other stats when I need fewer cures. (Phazon too expensive!)