There are some obvious important factors - most importantly the ability to extend my software (or pay others to do so) and to thereby ensure continued interoperability.
In general, because an increasing amount of our lives are dependent on software and there are far more users than developers.
Note crucially that we're not talking about all ability to make money (as we would be with Non-Commercial clauses); we're talking about the difference between the ability to earn money based on copyleft code (demonstrably non-zero) versus non-copyleft code (probably more).
