[charlesism]

It's okay only if you carefully explain to the candidate, before the interview, that you may be using his/her ideas (even if you do not hire him/her).

If you do so without consent, you make your company look like dirtbags:

"No, they didn't hire me, they just wasted four hours of my time, and stole all my product ideas. Why pay a consultant when you can interview a sucker..."