Fir some things it really gives a boost to the speed, to others it doesnt make any difference at all or is even negative.
As long as the clock speed is kept the same, there is no negative difference. There were with some dual CPU platforms as they generally had to use slower or registered RAM and had more conversative chipset timings to keep stable. However I dont think there are any performance losses with dual core platforms.
But otherwise, youre right, for most processes there is no speed boost. The main benefit is that you can run more than one intensive process at once.