Fireball has way more potential than a barb at low/mid levels if the caster is willing to grab a great big bunch of mobs, get at least yellow alert and then throw a fireball or two to take them out.
The overwhelming majority of casters at that level can't/won't do that. For everyone that doesn't, there's Haste. For everyone that does, they're probably already soloing that quest.
Edit: Also, AoE spells are significantly less powerful now that criticals are determined per monster rather than per casting. You will now need more fireballs to take out a group of monsters than you had previously.