Well you learn something new every day I guess
There are many advantages of multi-core processors over single core and using more than one processor on the same system, such as:
They are much more efficient when used with applications which take advantage of more than one thread. There are hyperthreading single-core processors which will allow a CPU to accomplish two threads with a single CPU, but an independent physical core will perform better than a "virtual" core as it were.
Even when running software not programmed to utilise more than one core, multi-tasking is much more efficient. Therefore you have room to perform CPU intensive tasks such as antivirus scans, encoding or games whilst performing other tasks without any noticeable slowdown. A single core processor would probably grind to a halt on the tasks I mentioned, let alone perform other tasks simultaneously. Even if that single core was say 6GHz, there still wouldn't be much room for other tasks without any noticeable slowdown. It would just get the single task done quicker than one with a lower clock speed.
Cache can run at a much higher clock speed than if you were to use two seperate processors due to cache coherency. This simply means that the cache for all cores is stored on the same PCB so cache signals do not have to travel off the CPU, therefore diminishing signal degredation.
As already mentioned, they are more energy efficient, so I won't go into detail about that one
