Holomark2 used all six cores on my computer for quite a few of it’s tests. This is, of course primarily a display tester but the CPU has to be used to support the rendering.
Also: I think that John’s explanation was over-simplified. While it should be obvious to most that even with just one processor Rhino can’t do anything at all with objects that haven’t yet been created, there are still opportunities, not realized in Rhino5, where some of the elementary geometry operations could benefit from multiprocessing. Drawing a line with a start and end point, or even a multi-control point wavy line will never benefit from multiprocessing for the reasons John stated, but there are many things that would, especially when manipulating very large objects. Choosing, designing algorithms and coding these takes time (months and years, not hours and days) and multiprocessing CPUs with enough cores to make it worthwhile are only now showing up at affordable prices and with enough support software to begin the work. So now some of the geometry tasks can be tackled. I would be very surprised and disappointed if V6 didn’t multiprocess several of the more computation-intense geometry manipulation commands. At least, based on what the amazing Rhino developers have done in the past, I don’t think they are just sitting around patting one another on the back and admiring their past work.
Due to the fact that multiprocessing requires some setup overhead, even tasks that would benefit from it on large objects might actually be slowed on small objects. This means that every command that might use it must first determine whether it would be worthwhile: more overhead. This check could be done quickly compared to the MP setup, but to be done as quickly as possible, may very well require some changes to the object representation in Rhino - a major overhaul. (I’m speculating here, not being a MP programmer.)
So my take on the answer to your question is that while 4 processors seem to be common and affordable these days and advantageous in general computer use, there is probably no big advantage to spending the extra money for 6, 8, 10, 12 cores to use for Rhino5. Maybe it will be a good idea for the computer you buy after the one you buy today.