The problem is that all of the performance tests are usually done with multithreading video encoding so not a fair comparison for most software.
Depending on what you do really. If you have a single thread exclusive workload, it might be cheaper to buy an overclockable Pentium or Core i3, count on the fact that there are only 30% or less of the chip active, and crank that clock sky high on standard cooling solutions. If it is heavily multithreaded you will need a high end Ryzen Threadripper or even a dual Broadwell-EP machine (dual-socket processors since Skylake are crazy.)
I pulled up that list noting Passmark per core per GHz, boiling down to almost just cache per core and instruction per clock difference across generations of Intel processors, which is barely.
With the multicore war just heating up investing in old server hardware doesn't seem sensible. It's all inefficient, hot and insecure compared to newer stuff. Patching can only get you so far, it's still a workaround for hardware issues and impacts performance even more. You don't need to do mission critical stuff to be badly hurt by a breach.
I am not using decommissioned server hardware as a server, I am using it as a workstation or even a home desktop. Market segment is being crossed here.
Besides, what was exotic server hardware not long ago is fairly mundane desktop territory today. With the added benefit of higher single core clocks on top.
It is natural for server technology trickle down to the mainstream.
Newer things often has fancies like warranty and R&D cost attached to it and it is on the high point of the tub curve. Those used server grade stuff doesn't have those fancies attached to drive up their prices, and being a few years down it is well in the bottom of the tub curve too. You do miss out on newer technology like energy savings and new instruction sets, but for a budget-limited machine build or a machine you intended to use privately for the decade to come, used server hardware cna make sense.
As of the single core clocks, overclockable Xeons can be pushed sky high just like their desktop counterparts and their lower stock temperature and lower power draw than the desktop chips of the same architecture can even affords them more overclocking overhead. As of the instruction per clock per core efficiency, the benchmark says it have barely improved for the recent decade.