If you conduct an interview, and at the end of that interview, you commit to letting the interviewee know “one way or the other”, please actually do this.
The radio silence certainly lets us know eventually, but feedback about why you've selected another candidate gives us info on what to do differently in our next interview.
Not to mention the fact that we've taken our valuable time & gas (or even more time & plus money on public transit) to come meet with you, while it takes 2 minutes of your time to type up an email or make that phone call.
Just because someone is looking for a job doesn't mean that we are unworthy of you keeping your word.