Very often the fastest chips for a given socket/chipset are sought after by people that want to upgrade older systems well after the chips have been out of production.
I sort of agree on the collectible idea. Though I think anything in the PCIe era is more or less equivalent. Aside from performance increases they are pretty much the same type of computers we've had for a while. You have to go back pretty far before things like AGP and PCI were the norm. I keep around my old GPUs and one computer with a working ISA and PCI bus just in case I have to troubleshoot something. (Not sure I have an AGP computer anymore, maybe my old PIII)
In recent months it is a suggestion I am making more often due to the price of buying new memory. If they've already got 16GB of DDR3, I say grab the fastest DDR3 chip and board they can get a hold of. Which is typically the i7-4790k and an LGA1150 board. So either $650+ for a new CPU, motherboard, ram. Or dropping in a used chip for a third the cost.