There is NO reason you would raise the number of HD on monsters to compensate for low level AoE blasting spells. The spells still deal 1d6 damage, with a reflex save for half. Unless you regularly throw huge mobs of 1hp monsters at your PCs instead of using legitimate encounters against CR 1/2 - CR 1 monsters (9hp - 15hp), evokers will not be coming anywhere near one-shotting whole encounters. Of course, Enchanters and Illusionists still will be, using Sleep or Color Spray.
Well, I was thinking more goblins and kobolds and things that normally use swarm tactics. Are you honestly telling me that you've never played a game where the DM just drew the next room on the map and dumped kobold minifigures onto the table?
But a single d6 has a good chance of eliminating a kobold warrior as presented in MM1, since they have poor abilities scores, Reflex Saves, hp, and energy resistance at first level. The DC for an average wizard's first-level spells is about 14, since any half-decent CO puts minimum 16 into INT.
Certainly, DMs will fill a room with Kobolds. That room was never meant to be a serious challenge anyway, in fact every DM I've ever had do that, including myself, is using it as free XP. Depending on how large the room is and how many Kobolds are actually in it, a Wizard using
any number of spells (including Color Spray and Sleep obviously) might one-shot the encounter. Moving fireball to 1st level simply makes it somewhat on par with other spells that are better
and have always been 1st level. This is the type of encounter that a Wizard is
supposed to shine in. It's the Fighter's job to efficiently take down the Grimlock waiting at the end of the corridor.
@Wotmaniac: Yeah, and Whirlwind Attack is a trap. Of course, a human Fighter can get Great Cleave, which is still a trap, but it'll cut through swathes of soldiers far more efficiently than Whirlwind Attack would at 1st level. A platoon of CR 1/2 warriors by the way, will have at minimum 6 hp each, and really some of them should have up to 10 or more, and most of them ought to have something close to 8. Unless we're talking about warriors with Int 1 on a featureless white plain 400ft away, the Fighter will slay them faster than the Wizard and with fewer resources expended. It will still take the Wizard at least two fireballs, perhaps three (half or more of his spells per day).