That was pretty much it. Fireball IS worth a level 2 slot because it does more than deal CL d6s, it does CL d6s to everything in a dungeon room.
I know this is one example about goblins, but I'm going to requote myself, here:
Actually, given a goblin warrior (2-9 HP), if you deal 1d6 damage, on a failed save, any one goblin has a 31.25% chance (15/48) of dropping/being disabled (assuming you roll for HP). If you take the average HP, you have a 33% to bring them to 0 or lower HP.
So, that means that a 1st level Fireball will drop about a third of the goblins in a room and seriously mess up the other two thirds. But, this is D&D land, and those other two thirds are at full offensive potential. Now, there are two things to take from this, that are both really important:
1) If you're fighting an EL 1 encounter, you're fighting three goblins. That means you just use a 1st level slot to kill
one goblin and mess two others up. Color me unimpressed.
2) Yes, it is technically possible that you could kill 10 or maybe even 15 goblins with one 1st level fireball, but that means, statistically speaking, that there are 20 to 30 more left standing! At 1st level! You don't fight those kinds of fights at 1st level! I mean, six goblins should be a 1st level boss fight, and even then, you're only killing
two.