[TallGuyShort]

The other side of this is that I, as a candidate, use open source as a litmus test for the employer. If the company contributes heavily to open-source, the culture is more likely to be compatible with me, AND it's legal for me to show my work to future employers. A lot of open-source work is done with corporate backing, so if one doesn't want to contribute outside of work, I'd suggest looking for an employer that will let them work on open-source projects, at least part-time.

[TallGuyShort]

Adding this a bit late for it to be an edit, but I'd add that I think it's unfair for employers to have open-source work as a de-facto requirement if they don't contribute to open-source themselves. Open source is also a chance for you to see how the company's existing employees comport themselves, how they work through problems, and what kind of day-to-day problems they're working to solve.