In my experience, a tour with a reasonably impressive nonprof can form the foundation of an entire career. Granted, the financial compensation often leaves much to be desired, but the investment pays off in spades when prospective for-profit employers see that name on your resume / portfolio. Participation in a worthy cause sends a very positive message about a candidate as a person.
This has been my experience. I was fortunate enough to have done a six-month project for a very well-known, high-visibility and pretty universally popular nonprof early in my career. Since then, I haven't gone a week without at least one headhunter soliciting new opportunities; at least half the projects I've worked on since then picked me up expressly because of that experience.