Whilst I am a single career developer, including all through college and university, I am also an interviewer who doesn't look for that same single focus over the long term. For one, I recognise that I have been very fortunate in choosing a career that has been good for me (and hopefully me being good for it too) but not everyone is as lucky.
It has taken me hard work but what I want to highlight is that many people put in hard work and still find themselves in the wrong career. This is not a bad thing unless they stay there. It has already been mentioned how valuable that experience can be. We should be judging not only by attributes and skills but also by potential. Candidates must also understand the balance between employer and employee. If you have potential you may have to start in a junior position but you may progress quickly. This does not always happen. Once whilst working on a project that was under resource strain (read they didn't expand the project to the appropriate size on time) we were offered a candidate for a senior freelance developer/designer that had no formal education nor training and less than a year previous was a kitchen/bathroom tiler. He could have been a great asset but he did not have the breadth and depth of experience for that role. If it was for a permanent junior or, possibly, intermediate role then I would have seriously considered him for interview.
Of course, some people change careers for other reasons such as a career coming to a natural end (e.g. sports, services, etc.) or the end of success or never achieving it (e.g. musician, artist or sport again).
-- Stop your grinnin' and drop your linen...they're everywhere!!!