The 2009 or 2010 or 2012 Mac Pro CPU can be upgraded to as much as a 12-core 3.46 GHz chip. With its internal bays and high speed PCIe slots, these older models are far from defunct, especially for video users (stuff several video cards into slots for video work).
With the 2013 Mac Pro priced not to sell (expensive), a CPU upgrade may be just what makes sense for some users. Downsides? No 4K display support as with the latest Mac Pro, and no Thunderbolt. Still, a CPU upgrade may be much more palatable than a $5K to $12K new model.
The graph below was derived a few years ago, but remains relevant in showing relative performance. Results vary greatly with which software and tasks are part of the workflow.