Bruce,
It's not "the" tool but it certainly is "a" tool used by various types of malfeasors to hide. It's an arrow in the quiver, along with the tools you raise (apologies for mixed metaphor). Indeed, there's a whole playbook, and we've only scratched the surface. In my experience, the "badder" the activity and the more sophisticated the "bad guy", the more levels and types of "tools" are used. Conversely, less sophisticated or devious bad guys tend to use fewer of these tools.
As for using free email services, I hope you don't think I'm hiding....
Greg