"I'm late, I'm late for a very important date..."

A job interview is like a first date - if you like what you see, you want more. If you're less than impressed, the date is over before it even starts. For politeness sake you might stick around and talk with the person for a short while but once you part company, you know you're not going spend any more time together - it's just not a good fit.
If you want to get to the second date/interview, pull out all the stops on the first one. Showing up on time is huge - the reason that "punctuality is the courtesy of kings" is because being late is rude to the person who is waiting for you. It gives them the impression that you think your time is more important than the person you've left waiting. While your friends may think waiting around for you is no big deal, an employer won't find this anywhere near as endearing. Wasting their time is the equivalent of wasting their money - there's no benefit to either party if you do so - especially this early in the game before you have anything of value on deposit with them.
Imagine your favorite actor invited you to meet them at a set time. Would you risk missing this once in a lifetime opportunity to see them by showing up late or would you plan to be early in order to avoid missing your big chance? Treat any job interview like that and I guarantee you'll never be late for one again.
BTW to my favorite Time Lord of all, David Tennant, feel free call me from the Tardis. I'll meet you anywhere, anytime, on time - I guarantee it!