I coordinate the extern program at my hosptial and agree with the other posters in this thread. A GOOD extern program (and they are not all good) can help your career -- but it is not a necessary thing. It is like many choices in life ... you have to look at the pros and cons for your particular situation and then make the choice that seems best for your particular situation, knowing that you will have to live with the consequences of your decision.
Many years ago, when I was an undergraduate, I had to choose between participating in an excellent extern program vs a scholarship
to study abroad. I chose the summer abroad. Many times, I have wondered how my career (and my life) might have turned out differently had I made the other choice. I would have learned skills that summer that would have helped me to avoid some career problems that still plague me. And you never know, who I might have met, what I might have done, etc. We can always speculate about "the road not taken" -- but it is only speculation.
I don't regret my choice to turn down the externship in favor of study abroad. My summer studying political science at Oxford was extraordinary -- one of the highlights of my life. While that experience has not had an obvious impact on my career (as the externship might have), it has helped to make me who I am as an educator, nurse philosopher, and scholar. Yes, I came back for my senior year of nursing school less "practiced" and less clinically skilled than my classmates. But that was the choice I made and I have been willing to stand by it all these years. While my nursing career has not been perfect, it has been OK and most people would consider me successful.
So ... if there is a good extern program available to a student ... and that student feels that it is right for him/her... then I would recommend taking advantage of that opportunity, even if it means a little sacrifice. However, if that is not the case, I wouldn't worry about it. There are lots of different routes to a successful career. We don't all have to follow the same path.