99% of people have more than one thing running at a time. I'm pretty sure everyone who has a multicore processor uses them all. Otherwise, they wouldn't have stopped making single core processors. Pretty much every processor available has 2+ cores now.
Perhaps drifting a bit off topic here - I don't dispute that most people have more than one thing open at once, but how many people have 4 or more programs open at once that max out all the cores? Even a lot of games these days don't multithread, and when they do they don't tend to do it particularly well (it may show all cores maxed out but in a lot of cases you've got things like twining going on that eat up processor power without really doing anything most of the time.) Regardless, with modern games the bottleneck is often in the graphics card anyway.
Yes, there are people who max out all the cores given to them. But for someone who plays a few games and browses the internet every so often, they don't come close to maxing them!