I agree on Spirit (which is why I capped it to no more than 50% of int), and admitedly I only had limited use for firewall (like you Hassat, I found running easier).
But must disagree with fireball.
Firstly, a maxed out fireball (15/15) can do up to 5k damage with the right build and buffs, and can do it over a very wide area. When you've got a swarm of minor minions running at you, magic missile just won't kill them fast enough because it will only hit them one at a time.
Secondly, it has a separate cooldown to MM. That means an extra 5k damage every 30 seconds in a boss fight, and this significantly increases your average DPS compared to MM alone.
Finally, there's many places where you can kill air targets from the ground (by shifting to human form). Human form has major advantages (health regen and access to potions), but MM just isn't fast enough to hit flying targets when they come in range - it mostly peters out before it even gets close. Fireball hits every time, and does enough damage to take out almost any flying opponent in one hit.
Magic Blast is an alternative to fireball, but it takes too long to do its damage, and doesn't have the area-based effect for when you're being rushed by minions, so I'd still recommend fireball as your secondary damage spell (MM as your primary).
