[chc]

I don't understand what you're suggesting the downside here is. They might decide not to take my code, lock it up in a proprietary system and profit from it without me getting anything out of it? How am I harmed by them not doing all that?

[minoru]

If your code was used by some company you may use that fact on the interview or to promote your project further, but if you are using GPL you are risking the chance to do so.

[chc]

This is a piece of software that they aren't willing to pay for (remember, you're offering it under a proprietary license as well) — how likely do you think it is to help you if they silently use your code and don't tell anyone?

You can't even "use it on the interview"†, because they just took it silently and didn't tell you they're using it.

† Incidentally, I hope never to interview with this hypothetical company that isn't willing to pay for value-providing software but would find the fact that I gave it away for free highly compelling.