Would Zapatero have won the elections if it weren't for the bombings? If 91% of Spaniards wanted their troops out of Iraq anyway, it seems to me as if the bombings wouldn't have made much difference to the election results. Just a thought. So I guess you could say that the bombings were a way for Al Qaeda to take credit for splitting the US-Spain alliance, which probably wouldn't have lasted much longer anyway...
I think the bombings benefited Zapatero. The polls said the things were 50/50, but the attack revived the frustration of the war and the show of manipulation.