spells that you would never want.

[Troacctid]

No, it didn't, because the enemies passed their saves (remember, the DC on Fireball and Stinking Cloud is exactly the same). At which point the rest of your party got to mop them up. And, no, Fireball didn't help, because the enemies you hand-picked to prove your point were dying in one hit anyway. A Gnoll has 11 HP. A 1st level Fighter one-shots it on a below average damage roll.

Hey, don't look at me, they're from the 6th-level random encounter table.

But let's imagine you were fighting an enemy which does not fold to a stiff breeze. Perhaps the CR 6 Annis. At 45 HP, it's still standing after two failed saves against Fireball. If it passes its saves you could very easily spend your entire allotment of 3rd level spells without killing it. Though, actually, it has Spell Resistance, which Fireball checks and Stinking Cloud doesn't. Let's try again. Maybe the enemy is a Bralani. Wait, no, that has SR and resists fire. It could be Girallon. That Fireball might actually do something against, though it can still fail two saves and live to talk about it (and it's Ref is actually higher than its Fort).

Fireball works on a greater percentage of enemies, statistically, than stinking cloud does, because poison immunity is more common than fire resistance. Attached to that debuff? Switch out fireball for Boccob's rolling cloud. Now you're doing the same damage, but you also get to daze them ~25% of the time, and for the kicker, half the damage is untyped. Not that you should be using either of them against only a single target...that's just inefficient. You should really be using a lower-level spell instead, like combust or force hammer or chill touch. (Or a different spell of the same or higher level, but in this scenario we're a 6th-level sorcerer, so we only have one 3rd-level spell known. One of the perks of warmage.)

Most damage spells are highly situational. If we take fireball as the baseline, the average CR 10 threat has 130 HP and a reflex save of 8. So our hypothetical level 10 wizard with lets say 24 intelligence has a DC 20 fireball, meaning that the enemy fails on an 11 or lower. So the fireball does on average 35 damage, and 45% of the time the damage is halved. Net average of 27 damage. That's five or so casts to drop him, without factoring in elemental resists or spell resistance. Doesn't look great. But fireball is an area of effect spell, meaning that it is incredibly inefficient on single targets. If we instead have an encounter of 2 CR 8 threats, which is the same total CR, the saving throws and hit points both go down, so the average number of casts to finish the encounter goes down to 3.3. If you take an encounter of four CR 6 threats, the number of casts required drops to 2.4. Keep in mind that these aren't your highest level slots, since at level 10 you have 5th level spells and could empower a fireball and that makes the average damage shoot up significantly. Back to the single target guy, you can maximize a scorching ray, and with a +1 bonus to caster level could easily be hitting his touch AC with maximized scorching ray for 72 damage, give or take 5% from critical hits and misses, and you're back to finishing the entire encounter in two casts.

This is a simplistic analysis against a foe with no SR or elemental resistance, but keep in mind these encounters should be fought with 3 other party members. I think a white room analysis that has a wizard able to solo kill many threats in 2-3 casts with direct damage with relatively minimal investment is pretty good. It's not like maximize or empower are really niche feats. Are there better options? Sure. Is damage particularly bad if your DM constantly hits you with single threats, or encounters that are substantially above your ECL? Yeah. If you don't have a way to bypass or overcome elemental resistance or SR, is it going to dig into these numbers? Absolutely. But I don't think you can say doing direct damage with spells is bad unless you're applying your damage spells unintelligently.

Empowered spellshards are pretty cheap too. And a ring of mystic fire is well within the price range of mid-level characters. If you switch from fireball to scintillating sphere, you could toss in a veil of storms for that extra kick while you're at it—electricity resistance is rarer than fire resistance, and you won't need to worry about burning the building down.