I run a P4 3.8GHz single core system as my main desktop, even though I do development with it. Switching to a newer Core i5 system will make it run 10 times as fast, but as the runtime on my huge (tuned) code base is under 5 minutes already, it really won't save me *that* much time compared to *editing* the code. It will save on build time, which is a boon to me, but even that savings is only due to the nature of my build process -- I do full instead of incremental builds.
I do plan on buying a new machine in a few months when I've saved the money, but my main point is that the hardware we use has been "good enough" for a good decade. It is the crappy software the people shovel out that drives hardware upgrades nowadays, not the actual need for faster hardware.
So it is to the hardware manufacturer's benefit that as much software as possible be absolutely incompetently written crap so that people will buy the latest shiny-shiny because their old one is "too slow."