And if you think about it, moving every 2 years is 15 different companies. If you stay a bit longer at the ones you like, say 5 years, then that's 6 companies in your employable lifetime. To put it another way you have 5-10 big projects that you can do in your (employable) life. (I think Jeff Atwood said something like that before).
This is of course for people that value what they do at work and want to make meaningful impact professionally in their lives. For other people family is much more important so it doesn't matter if they don't min/max the projects they work on.
