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Blasters can certainly be competent damage dealers - when it comes to dealing with large amounts of mooks, there's no one that can beat them. (As long as you don't run out of spells!) But here's the main issue, I think: for most martials, dealing damage is what they do. If you're a barbarian, chances are you want to run up and smash things for massive damage. With a blaster caster, they'll go down in half the time, but still be at full damage-dealing capacity for that time; with a battlefield control caster, they'll be partially disabled or singled out while you wallop them. Both are legitimate ways of dealing with enemies; however, if I'm playing a fighter, I like to fight! I'd just as soon not have the combat be over in 2 rounds, AND have to focus more on defense since the enemies are dealing relatively more damage.
Another thing to consider is that it's a caster's responsibility to make sure your teammates can successfully deal damage to tricky enemies. If they're invisible, you need to cast Glitterdust or Invisibility Purge; until you do, your teammates are effectively helpless. If they're flying, and your archer can't take care of it, it's up to you to Dispel their flight magic or Air Walk your teammates. True, you could simply blast the enemy... but then your team is just sitting around twiddling their thumbs, and if you go down before the foe is dead they could be in serious trouble.
TL,DR: Blasting is fine, but there are many things only casters can do. Don't neglect those responsibilities in order to blast!
PS: Remember also that bosses usually come with high saves and sometimes spell resistance to boot. Make sure you keep Enervate or something similar in your back pocket, or cast some good buffs so the martials can take care of it. These will be far more helpful than ineffectually pew-pewing at the boss while it slaughters your frontliners.