Yes, it is good to have a spare PC, however if you can't afford it you can always run a virtual machine for that kind of thing. You could even get a different hard drive and boot to it, or hell to just a different partition and dual boot. I like using virtual machines because you can have your main PC running at the same time. If you dual-boot to Linux, you can't use your main PC.
If you swap out hardware as often as I do, another advantage to having a spare PC is to troubleshoot parts and what not.
For a spare PC highest I will go is $600. The main Beast Build will cost me $6k