The downside is that it doesn't work on targets that don't have blood. The hex,draining, & debilitation spells make a lot of enemies much easier to kill, you could also invest into the blood magic tree for the spell blood wound, which does aoe damage, disables non boss targets & you can cast it without any worry about friendly fire damage.
If you want to be a support mage, I'd say stick some points into debuffing/support spells like the entropy line ,then some into healing & enhancements. Willpower increases the size of your mana pool, each point increases your mana/stamina by 5. Magic stat affects your spells potency, as well as how much health/mana you get back from using potions & your mental resistance. Also cunning if you want to unlock extra dialogue options, advanced skills & combat tactic slots.
You should put points into magic/willpower for mages mainly.
