Well the likelihood of you buying just a single core pc in the future is falling to zero quickly. Even dual core could be in danger of extinction.
The new Nehalem will be hyperthreaded quads - shows as 8 processors in Windows Task Manager. For a physical two cpu machine that's 16 threads straining at the leash... Intel have said that software companies should write for many threads -4,8,16,32...
back to your question though.. SW is apparently almost all a single core application because the solving is undertaken sequentially but there are some activities that will utilise a second core to assist with processes in the background.
I understand this amounts to about a 20% improvement overall although that figure would depend on exactly what you did and how often.
Unfortunately SW are reluctant to name directly those tasks that benefit however opening drawings,some surfacing, loading and saving assemblies, and simulation are supposed to be on the list.
Why exactly they are so secretive about this is a bit of a mystery...go figure..but there is lots of useful stuff they don't like sharing officially. Its a bit like the ineffectual help manual - it makes SW look like idiots but hey they do it anyway ...
The only work that truly benefits fulltime from multicore is rendering, PDM and analysis
So I guess you could say that SW doesn't use all cores but it does get a helping hand here and there where possible.
Will you see that as a 50% reduction in a benchmark? Wouldn't think so. even 20% ? not sure -probably not -occasionally maybe.
It would seem re-architecting SW to be more parallel is a difficult undertaking that may never truly come about. Probably there will continue to be small improvements from release to release.
Of course having say a quad for SW doesn't necessarily mean a lot of cpu cycles going to waste if you use other applications at the same time. I am sure there are useful things you can do while waiting for a long load or rebuild....HTH