The entire point of dual cores is that you can have the same speed at half the freq. and thereby save power.
Dual cores potentially draw less power, but that is far from the 'entire point' of a dual core.
From a marketing perspective, the chip makers want to offer you something faster, with the hopes that you will buy it. As there haven't been any major breakthroughs in increasing clock speeds in the past couple of generations, one way to overcome this 'lack of progress' is to package more cores on a die.