That seems overkill, and expensive. However, common Intel sockets being used right now are 1155, 1150, and 2011. A 2011 motherboard and CPU is going to run you at a lot of money. The lowest end 1155 and 1150 CPUs are the Celerons.
As for AMD, you've also got several options. Their AM3+ boards support FX chips, Phenom II, Athlon II, and I believe Semprons. AMD also has their APU line which works on the FM series boards.
I personally think, with so many motherboards having more advanced diagnostic options nowadays (not to mention you can do process of elimination) that having things like spare RAM and a basic video card, and a PSU would be more useful. Sure, having a spare motherboard might come in handy, but having motherboards for every possible computer someone could bring in would be very expensive. I wouldn't see a big point in buying spare CPUs, as it's rare to see a CPU failure.