I've found that using summoned creatures is more a tactical decision than one to beat up things. By the time you can summon something, it's usually less powerful physically than the tanks and less powerful magically than the wizards. So the real strength is in the miscellaneous abilities that they have, and just sticking an extra body on the battlefield. Summoned monsters love sucking up AoO's, flanking, aiding another, and taking some hits that the cleric doesn't need to waste healing magic on. In this way, you are preventing HP loss, like healing, and probably causing at least mininal damage. And of course, there are the always popular trapspringers and such.
The way I see it, damage dealing isn't actually a wizard's best area. They are good at hampering the enemy, and offense is usually best served by effects that take someone out of the fight, so the tanks can shred them later. Spellcasters only have so many spells per day, they can't afford to waste more than one on most enemies, hopefully one spell will neutralize more than one threat. That's why blinding the enemy rogue is better than acid arrowing him. If he's blind, he definitely can't sneak attack anybody, and he can't really do anything in combat or run away fast enough to make a difference. Sure, he has all his hit points, but Tordek can power attack to make up for the difference.