Hm. That's one of my favorite questions. I think it's only a bad question if it's judged as good/bad and viewed independently of everything else at the interview. Here's how I would interpret your answers:
I need the money
The candidate isn't afraid to tell you the truth, even when the truth isn't what you wanted to hear. Still, I vastly prefer working with people who are at least as interested in the job as I am. It avoids the conveyor-belt working environment, and we all have fun and are usually more productive. Probe the candidate to see if that's really the only reason. There's nothing wrong with it as a motivating factor by itself.
The commute will be much easier
Pretty much the same as above. (Though problematic if you're thinking of moving the office a ways away soon!)
I don't know if I do or not, this interview will go a long way in helping me make a determination
Well, I'm biased, since that's pretty much the answer I give. I suppose you should be wary that the candidate doesn't care about the product the company is producing, but anyone who thinks they can predict what a company will be like to work for solely on the basis of the product they produce is fooling themselves anyway. A company might be producing the coolest, most fun thing ever, and yet internally it could be run as a military dictatorship by the CEO and the candidate will end up spending 90% of the time on support or distribution or documentation.
I have friends who work here and think I would be a good fit
Same as the first two, with an added positive that friends tend to work well together and an added negative that people who tag along after their friends may not care about the job much; they just want to work with their friends. (And they're following, not leading, so odds are they aren't as strong as the ones you've already hired. Okay, weak odds. Just something else to dig into.)
Be sure to separate out what the candidate did at previous jobs, rather than what the candidate's team did.
I've researched your company, like the products and think my skillset is a good fit
Possible BS alert. First, check how good of a job the candidate did at the research. And make extra sure that this isn't one of those candidates that knows how to produce the right answer to any question, but can't actually produce. (It's the "smart but can't get things done" type.) Any serious design or coding question should highlight those types pretty quickly, though.
If none those red flags turn out to be true, however, then this answer is a good sign that the candidate will add energy and morale to the team.
I will kill you if you don't let me have this job.
Always a good argument.
I have 20 years of experience doing exactly the sort of thing that you're hiring me for.
This is a good thing, but again raises red flags -- why does the candidate want to keep on working on exactly the same thing? Is the candidate flexible, or are you going to end up in endless arguments about why you can't do it in exactly the same way as the candidate has done it before? The specific needs of your company should be considered adequately.
I will sleep with you if you hire me.
The candidate is your spouse. (And in the long run your sex life will probably be better off if you keep your work relationship and personal relationship separate.)
