There’s one vital advantage to the dual-core. With 2 CPUs at your disposal, if an aplication runs awry and starts using up all the processor’s cycles, you can shut it down without a full system crash because there is a second CPU sitting there to manage failing gracefully.
Now there is a much better argument to be made about whether the quad-core buys much performance over the dual-core in desktop applications not really designed for it. In many benchmark tests, the two perform neck and neck. But in others, the quad-core is as much as 1.9 times faster. So I’m running a quad core and happy with it.