The autodamage that Flaming Sphere provides isn't big damage,
It easily can be.
At second level, I once did more than 100 points of damage with Flaming Sphere in a encounter. One spell and the Wizard takes out 2 to 3 of the non-minion foes (damage-wise). That was easily 40% or more of the damage needed for that encounter, just from one spell.
That Wizard had Int 20 and a +1 item, so he was doing an average of 13 with the attack portion and 8.5 with the start of round portion. Even an 18 Int Wizard without an item would do 11 and 6.5.
My house rule for Flaming Sphere is a bit different. I have a generic must use an Action Point in order to sustain an Attack spell house rule (first time it is sustained, after that it's free). But, our Action Point system is different, so I'm not sure how well it would work with the standard Action Point rules.