I recently got an email comment on my post about "How Great Candidates Blow it in an Interview." (In my post I was talking about the need to intensively prepare for interviews.) An astute reader said "In some cases, especially when the person has cast a wide net (not preferred of course), the position is not exactly right for that person. As a result, the candidate might not present him/herself in a confident manner or the vibe is just not there."
This is precisely the reason I do not recommend taking "practice interviews" for jobs you are not keenly interested in. Many people suggest that interview practice is helpful, and I agree with that, but interviewing for the wrong job is the wrong kind of practice. It's like playing a casual game of wiffle ball or softball to prepare for a major league baseball game. Sure, some of the aspects of the game are similar, but the stakes are different, the "vibe" is different, and you are play acting and not sincere.
So definitely practice your answers to tough questions with someone, but don't interview for jobs where you are not a fit.