Raw clock speed is raw clock speed, but at a certain point the resources needed to push the clock speed higher are not economical. The chip runs too hot and draws too much power. So adding extra cores makes more sense as the power demand will go up, more or less. linearly with processing power. It might even make sense to slow the default clock speed to be able to add more cores. Your clock speed will go down, but you processing speed, (ie # of instruction completed per second) will go up.
Yes there is some overhead in having to parallelize stuff, but even with that your still going to be ahead of the game.