It's not about 8 dice for 9 mana, it's about the 9 mana Hammer for the 5 mana Hand. Repeat three times (Hand is used in bulk), and the Hammer mage is down a whole turn of channeling.
And that's setting aside the other problems: 3 Hands is 6 points (or 3!), 3 Hammers is 12 points. Most books just don't have room for that many attack spells. The Hand player can go to 6 Hands, too, which won't be answerable with Hammers without the additional cost of wands.
One bad possible scenario is that a Priestess spends the first three turns casting Hands and the opponent spends the first 3 turns casting Hammer from a Wand. The Priestess starts turn 4 at 20 mana, and opponent runs out of mana on turn 2.