I've got some dual 8 core Xeons servers at work, and really only a huge database or a VMware server can use that many cores. Something like a huge rendering or scientific calculation workstation could, too. But as Xeon boards are almost all dual processor, you are usually cheaper getting two 4 cores instead of one 8.
And just a minor correction to amuffin's comment, quite a few of the Sandy Bridge based Xeons are not hyperthreaded. Especially the 8 way capable chips.
If you really have an app that can burn 8 cores, the FX-8150 will actually peform quite well there.