I think damage spells get a little unfairly bashed on the optimization boards. They are a little underpowered, granted, but it's hardly character suicide to focus on them. One key thing is the synergy: you deal hp damage, the Fighter deals hp damage, together you take down the enemy faster. If the wizard's tossing save or dies, then the rest of the party is basically working on a different mechanic. If the enemy makes 5 of those saves in a row, the wizard just spent the combat doing nothing while the others slowly killed the guy. If he had been blasting, they might have finished in 3 rounds or whatever. If the save is failed at some point...anything else the party did doesn't matter any more.
This isn't to say damage is the only way to synergy with the party to do better in combat...I love battlefield control and debuffing to help the other people in the party dispatch them. If the party's mostly ranged and the enemy is not, then laying down tons of effects that halve movement to entrap the enemy in a mire is totally a good way to synergize, for example.
Anyway...I think the main "problem" with direct damage is two-fold. One, everyone else can do it anyway, so shouldn't you cover something else? And two, other folks do it better. A raging barb or well built fighter will simply do more damage than a caster. Or at worse, barely less damage, but with the nifty "I can do this all day long" class feature. Blasters CAN do more damage against groups with area spells in terms of total hp dealt, but...3E tends to have small groups of enemies in combats, often spread out or mixed into melee with friendly units. More importantly, there's an absurd amount of reward in focused fire tactics. If you can do 10 damage to 3 enemies but Bob the Fighter can do 15 to 1 enemy, unless that 10's enough to kill them all, Bob's being more efficient. The sooner an enemy drops, the sooner the enemy has one fewer action per round to hurt you. So ummm...I guess the "action economy" would be reason #3...
Anyway, Id say that's the main problems with direct damage casters in 3E. Perhaps honorable mention for the quickly inflating hp as level increases (the fact that for every +2 con means you gain an extra +HD hp on top of the normal level up amount means hp does not climb linearly, while most evocations remain a piddly d6/CL).