Aside from a few select software titles that nobody should be using on a laptop anyway, there is really nothing out there that benefits from the additional cores. There is a lot of research to back this up too. Tom's Hardware did a great writeup showing that for normal usage, a quad actually slowed performance for several common uses of a computer.
The most popular software that utilizes multi-core processors is benchmark software. Yes, an octa-core desktop will score much higher than my mac laptop in benchmarks, but the .01% gain in a few "real life" situations is not really worth it. However, you will get bragging rights and street cred though. It has been stated by others often and I too believe that it's kind of a marketing ploy for the most part. Like, here is a car that goes 150mph, but for $500 more, it can go 200mph. When do you plan to drive 150 mph anyway? The fastest roads only allow 70mph.
But...if you are making the next Matrix movie, that's a different story. But who walks into an Apple store with the intent of buying a laptop to make a feature length Hollywood film with?
I had an overclocked Intel Q6600 quad for a long time and never once maxed it out. Now my desktop is an AMD tri-core running at 3.2ghz and still have yet to come close to maxing it out with CPU intensive work.
The real bottleneck on a laptop is the hard drive. Save your money and buy a decent SSD when they are affordable.