My guess is that this was the cheaper option. It only cost Joel a couple of days and several hundred on the hard drive. What it did not do is interrupt his programmers. As Joel has already described, he is now a grunt worker. His job is to make it as easy for others to do their jobs. Their time is probably worth 10 to 100 times his, because they are the one actually making something that directly results in more sales and money for the company. So all Joel had to do was save a couple of hours of work for the programmers and he would come out ahead.
Plus these additional side benefits:
The programmer who was complaining does not feel like he is being ignored.
Joel is now going to upgrade most of their computers, making it better for the entire company.
So lots of warm fuzzies to go around, leading to happier programmers, leading to a better product.
The cheaper option would be to run a compile and open any of the many different programs in Windows that tells you whether you're using lots of CPU, RAM or IO-waiting.
Then you go AHA! I'll spend money on CPUs or making my compiler/build parallel.
Plus these additional side benefits: The programmer who was complaining does not feel like he is being ignored. Joel is now going to upgrade most of their computers, making it better for the entire company. So lots of warm fuzzies to go around, leading to happier programmers, leading to a better product.