No doubt almost everything nowadays has at least two cores, and at least with Intel you have hyperthreading on top of that. Even my 7 month old Kindle Fire has a dual processor. Software is gradually catching up with the hardware, but there's still a lot that doesn't multi-task. I don't play the fast action games; I understand that some of them really multi-task well, but outside of that and doing updates, I see a whole lot of times when my CPU meter hits 50% with little disk activity and just sits there for 15 - 20 seconds. It seems like installing Office 2010 (64 bit) had a good bit of this.
My experience with multi-tasking goes back to the 1960s with mainframes when we tried to get the perfect mix of commercial and scientific jobs running on mainframe computers. The commercial jobs were I/O bound and the scientific ones were number crunchers, so a mix got good use of the expensive resources.
Getting back to PCs, most still have a single hard drive which operates in milliseconds when the RAM and CPU operate in nanoseconds. For those not good with numbers a millisecond seems fast to us humans, but a nanosecond is a MILLIONTH of a millisecond. Given a little thought, it's obvious that the speed of the hard drive makes it the bottleneck in most desktop and laptops. SSDs will eventually eliminate the seeking and rotational delay we have in HDs, but they still operate in milliseconds. Caching with lots of RAM or even in the hardware helps, but we still have the factor of a million in speed differences, so the developers of storage devices have got their work cut out for them. I read the gee-whiz stuff about quantum computers, but right now that seems more like a fantasy than reality. Still we've seen incredible progress in storage devices since the early days of disk drives. I remember paying something like $265 for a 5.25" floppy drive to go on a Commodore 64. Going back even further to the mid 1960s my first disk drive experience was an IBM 1301 that was the size of a big refrigerator, had platters about 2 feet in diameter, and held all of about 50 meg. When that sucker would seek, it would shake the floor.