They definitely will say that they don't care how popular is, but that is complete bullshit.
37signals is popular because of Rails, the more popular Rails is, the better for 37signals products.
As a Rails programmer, the more popular Rails is, the more worth are your Rails skills. Simple as that.
For a similar reason, the infamous OS wars are not as pointless as they seem. If I invest in Linux or OS X, I also have an interest to protect that investment, which means the OS in question should keep a minimum level of popularity.
"As a Rails programmer, the more popular Rails is, the more worth are your Rails skills."
This is still subject to the laws of supply and demand. Your Rails skillz will be valuable at a time like now - where there are a healthy number of startups using the framework for development, and a sufficient shortage of experienced Rails devs to hire. However, if Rails became popular enough that everyone decided to learn it and the number of available jobs didn't shift in proportion, you'll find your skillz less of a commodity.
37signals is popular because of Rails, the more popular Rails is, the better for 37signals products.
As a Rails programmer, the more popular Rails is, the more worth are your Rails skills. Simple as that.
For a similar reason, the infamous OS wars are not as pointless as they seem. If I invest in Linux or OS X, I also have an interest to protect that investment, which means the OS in question should keep a minimum level of popularity.