I think it depends on what you do with your computer. Some applications can certainly use more cores, but others won't and you might not notice much (if any) speed increase from the extra cores.

Here's a good look at multi-core processors that you might find interesting:

Multi-core processor
http://en.wikipedia.org/wiki/Multi-core_processor

"A multi-core processor is a single computing component with two or more independent actual processors (called "cores"), which are the units that read and execute program instructions.[1] The instructions are ordinary cpu instructions like add, move data, and branch, but the multiple cores can run multiple instructions at the same time, increasing overall speed for programs amenable to parallel computing. Manufacturers typically integrate the cores onto a single integrated circuit die (known as a chip multiprocessor or CMP), or onto multiple dies in a single chip package.

Processors were originally developed with only one core. A many-core processor is a multi-core processor in which the number of cores is large enough that traditional multi-processor techniques are no longer efficient[citation needed] — largely because of issues with congestion in supplying instructions and data to the many processors. The many-core threshold is roughly in the range of several tens of cores; above this threshold network on chip technology is advantageous. Tilera processors feature a switch in each core to route data through an on-chip mesh network to lessen the data congestion, enabling their core count to scale up to 100 cores."